How far can you realistically get without studying openings/tactics/endgame etc?

Hello friend, You can be a Intermediate player that is how far you can go. By playing a lot of games and practicing you can be good. Some people say it is not necessary to study or to read books. You can get better just by playing a lot of games and by practicing. But, it is necessary to analyze your games on your own and make conclusions about the game by yourself. 

I think what prompted this question was checking out this forum for the first time. I was seeing lots of names of openings being thrown around and came to the conclusion that there must be a side to the game I'm missing with my approach. In reality though, I'm already "studying". I'm analysing where I went wrong when I lose and taking those learnings into future games.

Nah, openings don't matter much, although sure, the fact that there are so many names for so many moves is typically intimidating / information overload for new players.

There is a sort of secret knowledge (if you want to call it that) that lower rated players are missing, and it's more about being able to read the position based on pawn structure, and knowing endgames.

This tends to stay "hidden" knowledge for lower rated players, because it's 100% useless until you can master not giving away pieces for free (or to 1-2 move tatics)... but soon after that's mastered, players will sometimes get losing positions and wonder how or why they suddenly have no good moves.

I feel like I'm getting there in rapid. Losing a few games with no clear blunders. Where can one acquire this "hidden" knowledge? :-D

Kmoch's book "Pawn Power in Chess" is often recommended. Soltis' book "Pawn Structure Chess" is pretty good too. The wiki article is like a synopsis of the Soltis book:
https://en.wikipedia.org/wiki/Pawn_structure

I think it depends on your own mental capacity and the amount of effort/brain power you're putting into it. If you're using ALL of it, as in, you're lucid dreaming about Lucena positions, I think you can get further than Paul Morphy and the likes did centuries ago which I assume they did purely by analyzing OTB positions.
By going into depth and studying some book knowledge you can expand that very quickly though. I don't understand the reluctance to take a few shortcuts.

Forget the openings. Study the endgame. Join a chess club and play OTB. I'm 62 who learned chess very late and just started playing again after not playing since I learned how to play at 43. My rating will be just a few points under 1900 as soon as my last tournament is rated. You can realistically go a long way if your dead serious about it.

I'm in my 50's and feeling a bit "trapped" by the barrage of chess courses, books, videos focusing on openings. It feels like a lot of pressure, and definitely not fun!

So, I'm curious about your experience. Now that you are around 1900 (congrats, btw!), your opening moves must be quite good. I'm wondering if you "accidentally" found specific openings in your trial-and-error process? In other words, looking back on your games, did you in fact play specific openings without realizing it?

Also, to what extent did you "not study openings"? Clearly you have to make a choice on move one, with either color. Do you just pick one, 1.e4 for example, and just stick with it and experiment wildly? Or, do you mean by "not studying openings" that you didn't go deep into theory? Did you know any main  lines, even just a few moves deep?

I'm approaching 1600 in daily chess, and would love to keep improving without the fairly boring opening study (memorization...).

That all makes sense, especially the part about understanding the "why" behind specific openings, and finding recurring ideas of openings. This info is very difficult to come by though, since most courses and videos/books seem to be stuck on the specific lines, or memorize this or that opening trap etc.

I do find myself having much more fun playing in open positions, lots of tactics, strong bishops etc. I've just switched to 1.e4 from 1.d4 and already enjoying the wild games much more. I do find in my daily games that opponents often seem much more prepared, especially in the opening, and I often find myself fighting to get out of trouble. In some ways this is very enjoyable because it forces me to really dig deep to find a way out (of course a big successful attack is even more "enjoyable").

I much prefer longer time controls. I never play anything under 20min, and mostly stick to Daily.

My Daily is 1545 and rapid is just under 1200 (but probably under-rated due to not playing rapid for a long time while daily has gone from 1200 to 1500+).

As black I've had a blast playing the Skandinavian, but still haven't found a real keeper against 1.d4.

My goal is to continue to grow and improve with a modest time investment (3-5 hours/week). For whatever reason 1800 sticks out as a goal. I mostly play online and not really interested in any "serious" otb play.

currently 1828