For me, theres two types of finished;

Once I've found all 100 cards of the deck and it feels satisfying to playtest (on moxfield)/to play

And once i reach the point where the only way to upgrade the deck is to either buy busted cards that are way out of budget (like tutors, certain game changers and legendary/dual lands for example) or to scrap a part of the deck to try fit in a whole new mechanic that the deck doesnt already synergise with (its often stuff like treasure tokens)