Practice, practice...
It is like in any sport. Use it or lose it.
How to stop getting worse at tactics

Practice, practice...
It is like in any sport. Use it or lose it.
It'd help to know your opinion on how much one should practice what types of tactics daily on average, depending on rating. For example, I've been practising only the hardest mates in 1, 2 and 3 on Lichess for the past 5-ish days, but at this point I tend to solve around 500 right and 5 wrong, which is keeping me stuck on my current rating of around 2700. Is there a rule-of-thumb sort of a limit at which I should say "okay, that's enough for now, time to switch it up"?

Play games, when you play games you start seeing the overall balance of every sound position. When the opponent breaks this balance you start looking for limiting moves, sometimes these are forcing sometimes not, but tactics trainers are not the de la Maza shortcut so seem to think they are. It depends on how your brain understands the game.
I think it's easy to get better at tactics just by practicing puzzles for the most part... but the real problem is, it does not stick. You cannot even stop training yourself when you reach a certain level, because you start forgetting. Chess skills deteriorate, and it's pissing me off.
I wonder if there's a level at which you just never forget some tactics. Someone once said something along the lines of: "A beginner practices until he gets it right; a master practices until he can't get it wrong". Is it really possible to learn this power? How much more do non-experts need to put in to become tactical masters, by this definition? Sure, you need to study all the aspects of the "great [chess] mystery" to get better overall, but tactics seem to go a much longer way than anything else... which makes me genuinely sad, because I keep needing to have to relearn and then maintain that stuff.
It took me thousands of losses and dozens of failed attempts of a tactic to finally get it right. But the moment I got that one opportunity right, I realised in that moment: training tactic helps (albeit in the long run).
I'm sure the most basic of tactics (hanging pieces count!) can be retained in minds for years.
Practice, practice...
It is like in any sport. Use it or lose it.
It'd help to know your opinion on how much one should practice what types of tactics daily on average, depending on rating. For example, I've been practising only the hardest mates in 1, 2 and 3 on Lichess for the past 5-ish days, but at this point I tend to solve around 500 right and 5 wrong, which is keeping me stuck on my current rating of around 2700. Is there a rule-of-thumb sort of a limit at which I should say "okay, that's enough for now, time to switch it up"?
Never mind, I failed to read this.
But still, I'm sure you can still improve beyond this.
My tactics rating is not too high either, so you need someone more qualified to give you more sound advice.

To me the key is becoming ever more vigilant and keep your eyes open to more possibilities. It is amazing on analysis the kind of stuff computer will reveal and I don’t mean some convoluted line 15 moves deep that you will never realistically find, but just a candidate move sequence that would be obvious to you had you just considered it.
So basically, a kind of Guess-the-Move? I can get behind that!
I think it's easy to get better at tactics just by practicing puzzles for the most part... but the real problem is, it does not stick. You cannot even stop training yourself when you reach a certain level, because you start forgetting. Chess skills deteriorate, and it's pissing me off.
I wonder if there's a level at which you just never forget some tactics. Someone once said something along the lines of: "A beginner practices until he gets it right; a master practices until he can't get it wrong". Is it really possible to learn this power? How much more do non-experts need to put in to become tactical masters, by this definition? Sure, you need to study all the aspects of the "great [chess] mystery" to get better overall, but tactics seem to go a much longer way than anything else... which makes me genuinely sad, because I keep needing to have to relearn and then maintain that stuff.