I just started the game a few days ago, if I leave one area to the next, will I be able to revisit and 100% them later, or do I have to full reset to 100% every area?
Can you please clarify - are you talking about the moment before you interact with Vestia at the center of Sablee (in front of the blue seal)? If yes - you won't be able to return to Sablee once you progress to the next area, so 100% every available location before that.
UPD: And that is the same for other areas you will go through. See a warning with confirmation? - Better go back and clear everything.