Enemies never stop becoming as powerful as you;

There are certain types of enemies per caste, that level with you;

Mythic Deathclaws, Super Mutant 'Warlords', 'Deathskull' Radscorpions etc.

I'm 190 on current character, and these guys are always my level.

There's usually 1 with a group of guys, but way more on survival, which is all I play, so I kinda forget how abundant they are in other modes.

You can use the Awareness perk to see which ones though, it shows creature level.

Loot, becomes, well, depends how well you know the game. The best weapon in the game can be had at like level 2, if you know what it is, where to get it and why it's so damn good (you also need Nuka World, but it's not 'in' Nuka World) and so on. You definitely stop caring about 'gear' at a point, my 190 well passed that, long long time ago, but I still 'loot' cause I love building, so I need supplies etc. and I go around trading and the like getting shipments etc.

Perks stop around 286-ish iirc. There's really nothing left I care to take at 190 already, and has been for a while, but it's whatever, I mainly play to build and shit, so depends what you play for too.

If you're the type that likes to bum-rush quests, you could be done by level 20.