It’s a bit of a long shot, but you might enjoy traditional roguelikes. At least, I kind of felt similar and then found them to be what I was looking for. How they fare for your individual points, roughly sorted from pro to not-necessarily-pro:

• I don’t like the feeling of being OP, I want to struggle through the end
• I hate grinding

Traditional roguelikes have brutal difficulty and grinding is effectively not a thing you can do.

• using items feels like cheating, so I tend to just use character abilities (I will heal if needed); I’d rather “git gud” than buy and use items
• I don’t like loot

The brutal difficulty forces you to use items. It won’t feel like cheating, but rather the only way to progress.
And there’s no way to sell loot, so basically whatever you find, either it’s new equipment or you just leave it there.
This also means money is extremely tight. You won’t be able to buy a hundred cheese wheels to counter-heal through encounters.

• turn based combat (tactics) is generally boring, but I do like puzzles, so that can make it acceptable

Very much turn-based, although not JRPG-style (which I dislike, too) and the games do generally feel like large, somewhat less strict puzzles.

• I don’t care about leveling up/character builds, it feels like a chore; abilities also don’t interest me

Well, this one’s a bit tricky. Traditional roguelikes are kind of all about that, moreso than RPGs. Because a death (or a win) resets your game progress, they can rapid-fire progress at you.
On the plus side, this is all part of the larger puzzle. It is not just a chore, but rather key to beating the difficulty.

I guess, I should also point out that by “traditional roguelikes”, I mean games that are actually like the 1980 game Rogue. So, don’t expect hyperrealistic 3D graphics. 🙃

Here’s two games that are relatively popular + beginner-friendly:

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