How do you approach 100% completing a game?

I was wondering how people approach 100% completing a game. Do you finish the game first and then immediately try to complete it 100%, or do you wait a bit after beating it before coming back to play for full completion?

I’ve been thinking about 100% completing Resident Evil 4 (2023) since I just beat it today, but I’m having a hard time deciding because I have other games I want to play. Plus, I’m only one achievement away from 100% completing Persona 3 Reloaded.