No one can tell. But if you put in the time it's probably possible for most people to get a 1800 rating. But it won't be handed out to you, you will have to put in the hours for it. If you spend that much time on chess and spend it good(!) I wouldn't be suprised if you would get to expert level. But again, it's not just the hours, it's how you spend the hours.
Think of a musical instrument, if you are 'practicing' hours without a metronome, you probably aren't practicing at all and won't see as much improvement as the guy who plays 10 minutes a day. With a metronome and full focus.
What rating can I ultimately hope to achieve?

We never know for sure we just work on it and see what we make of ourselves. You like me I think so we should learn together and maybe both improve by leaps. Play games against each other, work out imporvements, discuss the games afterwards and things like that. Message me, it could be excitingly fun and improve a lot!

Not that rating, in and of itself, is super important, but Ive always been a sort of "all or nothing" kind of person and it would bother me to think that my knowledge and understanding of chess somehow has this inevitable cap on it just because I didn't happen to start playing when was 6.
I started playing seriously just a few months ago. Im 28. I currently spend ~32 hrs/wk on chess, whether its reading books, playing live/correspondence games, tactics, etc.
If I work hard and stick with it, am I doomed to plateau in the range of mediocrity? Is excellence impossible?
What is it that typically keeps people from progressing when they hit a certain age? Is it the demands of adult life? Is it that they haven't trained their brain nearly from birth and its just too late? What causes this?
If you maintain 32 hours per week of study, there is no reason you couldn't become an expert or master.
The point that Wilbert_78 makes is a good one. Your focused time is much more important than the total time you spend.
How long do you spend on each of your moves? It seems like this latest game your tactics training was missing:
Black had been playing for mate since move 10. The position after 21..Ng4 screams out that you have to be careful to avoid getting mated. The rook sacrifice for your knight on f3 seems pretty obvious.
How long did you spend on Bxh8? You had an alternative that would have saved the game. Better spend more time on the Tactics Trainer.

Spend less time on chess, but make it effective time.
30% Going through games played by nineteenth century masters (you might go through the games of Greco first--it won't take long)
20% Endgame study
20% Tactics exercises
15% analyzing your own games
10% playing
5% going through games played since 1900
Spend less time on chess, but make it effective time.
30% Going through games played by nineteenth century masters (you might go through the games of Greco first--it won't take long)
20% Endgame study
20% Tactics exercises
15% analyzing your own games
10% playing
5% going through games played since 1900
I might be wrong but I want chess to be fun not something akin to work. I like I_Am_Second idea, do what you're in the mood for and do it with gusto!

Spend less time on chess, but make it effective time.
30% Going through games played by nineteenth century masters (you might go through the games of Greco first--it won't take long)
20% Endgame study
20% Tactics exercises
15% analyzing your own games
10% playing
5% going through games played since 1900
I might be wrong but I want chess to be fun not something akin to work. I like I_Am_Second idea, do what you're in the mood for and do it with gusto!
If you want it to be fun, then I recommend 100% going through nineteenth century games. But, if you pursue excellence (as you suggest in the original post), then train you must.

@OP : assuming you haven't played against rated players in your youth : 1800 is certainly within reach, 2000 is a reasonable yet not easy goal, 2200 is a very serious challenge.
Anything above would be damn impressive.
Spend less time on chess, but make it effective time.
30% Going through games played by nineteenth century masters (you might go through the games of Greco first--it won't take long)
20% Endgame study
20% Tactics exercises
15% analyzing your own games
10% playing
5% going through games played since 1900
I might be wrong but I want chess to be fun not something akin to work. I like I_Am_Second idea, do what you're in the mood for and do it with gusto!
If you want it to be fun, then I recommend 100% going through nineteenth century games. But, if you pursue excellence (as you suggest in the original post), then train you must.
You confuse me with Curlaub or someone. But yes fun is most important, it is game after all.

One thing is certain though, if training and practice feel like some that is not fun, you are not going to become a chess expert. Just as if playing the piano feels like a task instead of pleasure you will probably be not playing any solo's in a philharmonic orchestra ;)

If you maintain 32 hours per week of study, there is no reason you couldn't become an expert or master.
The point that Wilbert_78 makes is a good one. Your focused time is much more important than the total time you spend.
How long do you spend on each of your moves? It seems like this latest game your tactics training was missing:
Black had been playing for mate since move 10. The position after 21..Ng4 screams out that you have to be careful to avoid getting mated. The rook sacrifice for your knight on f3 seems pretty obvious.
How long did you spend on Bxh8? You had an alternative that would have saved the game. Better spend more time on the Tactics Trainer.
Yeah, that was just a case of underestimating my opponent. I got a little too comfortable and wasn't looking as closely as I should have been.

I looked at one of your games and you overlooked a mate in 1:
Holy crap, I didnt even see that after going through the game again after. Geeze, what could have been. I've always struggled with tactics, but the road to mastering the is pretty long and I still feel Im doing pretty well for only having been playing for a few months. Im not saying Im ok with where I am. I definitely need to improve, but Im happy to carry on and keep training. Ill get there.
Do you have any advice for improvement in that area? I spend a good amount of time on ChessTempo and Lichess tactics, but I always hit a wall and I cant get past about 1300 on CT and 1500 on Lichess.
Im working through all the books on the reading list at /r/chess. Im reading Logical Chess Move by Move at the moment, but tactics have been an issue for me for a while and Chess tactics for Students is definitely next on my list. In fact, I may set Logical Chess down for a while and start on Chess Tactics pretty soon since that seems to be a more tangible and immediate hinderance.

32 hours per week is an EPIC amount of time and very admirable
The training program I recommend to my students does not require anything close to that, so you'd obviously be able to do that.
If you actually trained for 32 hours on the right program, you'd see real results, fast (in terms of weeks/months, but remember we're talking 32 hours/week!)
There's no limit rating wise based on age. The steps to get from Class D-> A and from Class A to Expert and from Expert to National master, etc is not magic and hard work, it's how you're training that matters.

I looked at one of your games and you overlooked a mate in 1:
Holy crap, I didnt even see that after going through the game again after. Geeze, what could have been. I've always struggled with tactics, but the road to mastering the is pretty long and I still feel Im doing pretty well for only having been playing for a few months. Im not saying Im ok with where I am. I definitely need to improve, but Im happy to carry on and keep training. Ill get there.
Do you have any advice for improvement in that area? I spend a good amount of time on ChessTempo and Lichess tactics, but I always hit a wall and I cant get past about 1300 on CT and 1500 on Lichess.
Im working through all the books on the reading list at /r/chess. Im reading Logical Chess Move by Move at the moment, but tactics have been an issue for me for a while and Chess tactics for Students is definitely next on my list. In fact, I may set Logical Chess down for a while and start on Chess Tactics pretty soon since that seems to be a more tangible and immediate hinderance.
On those other tactics sites, do they provide statistics? What percentage are you getting right? Do you guess quickly or think until you are 99% positive that you have the right answer? I think it is much more important to completely solve the problems than how long they take you. If you can't see a good line, you should keep trying and avoid the temptation to just guess. As you get better at solving, your speed will eventually improve.
When I look at my statistics here for Tactics Trainer, I can see the graph of slow and steady almost painful progress point by point, which I can throw away with a few quick guesses. I like the fact that bad guesses are sharply penalized.
They are looking for forcing moves that win pieces or result in mate at our levels. They are not looking for ways to leave a pawn structure subtly weakened.
With 32 hours a week, you should finish the Chernev book this week, as well as another book or two of that type. I don't remember them being difficult reads. Books like that are like bedtime stories. Enjoy them. Let them open your eyes. You don't have to memorize them.

" Geeze, what could have been."
Refuge in audacity. When some huge egregious error is made we overlook it because the idea that our opponent would do such a thing seems very preposterous. The opponent is trying to win so why would they make such a careless move after all?
For Chesstempo look at the patterns you're weakest then do problems that are based on those. You may need a book to highlight why such moves work. Quiet moves and defense tend to be the biggest pain on Chess Tempo due to the expectation of a big capture (especially an exchange sac) being the correct move. I personally use CT-ART 5.0 (has CT-ART 4.0, Beginner, and Mating Patterns included)

Tips
1. Never count playing blitz/bullet games or going on the forums or participating in groups and teams ect. as part of your training. I suggest you avoid those to a large extent.
2. Actually spend 32 hours. I know personally it's easy to set goals and then fall from them, but if you do spend 32 hours a week in a good training program your off to a good start!
3. Get a coach- and then ask him to give you advice on how to spend your time, if your really serious about becoming a master a coach is the way to go.
That would be my advice.
Not that rating, in and of itself, is super important, but Ive always been a sort of "all or nothing" kind of person and it would bother me to think that my knowledge and understanding of chess somehow has this inevitable cap on it just because I didn't happen to start playing when was 6.
I started playing seriously just a few months ago. Im 28. I currently spend ~32 hrs/wk on chess, whether its reading books, playing live/correspondence games, tactics, etc.
If I work hard and stick with it, am I doomed to plateau in the range of mediocrity? Is excellence impossible?
What is it that typically keeps people from progressing when they hit a certain age? Is it the demands of adult life? Is it that they haven't trained their brain nearly from birth and its just too late? What causes this?