I never 100% these games - far from that.
And I also never self-insert or even identify with the MC - that’s why I favor the games that are written in first person - unfortunately, that’s uncommon. I’d rather know the thoughts of the MC from his perspective than be adressed to as if I was the MC. But well, obviously I had to get used to that. Still, that’s besides the point.
So, as for the actual question, I make my MC, then make it my goal to “help him achieve happiness” - so I basically try to get what I think is the best route, with the most suited romance for him in my opinion. And yes, I say him, because all my MC’s (and chosen ROs) are male.
I basically consider whatever route I’m doing as canon, so usually, when I replay (and I replay a lot!), I make the exact same choices, for the genuine pleasure and enjoyment of reading the story again and again, and without stressing out about the choices.
On veeeery rare occasions, I want to explore a different romance. Unless something in the game prevents me from doing it like I want, I’ll keep the changes in my new playthrough strictly to romance. That means every single choice I can do the same, besides romance, I’ll still do the same. But obviously if romance plays a big part in the game, that could change the entire story. For example, you won’t see me replaying Wayhaven Chronicles with a different RO from the first one I’ve chosen, because the game is truly romance driven. On another hand, I find myself trying various options in Samurai of Hyuga, because romance, while important, isn’t the focus. My MC is still exactly the same, aside from the romantic interest.
Now, if I happen to get a bad ending, or rather an ending I’m not satisfied with (sometimes a good ending is still not satisfying, and sometimes a bad ending actually is), then I’ll probably replay until I get an ending I like, changing small things here and there, as to keep my character true to himself, but still get another outcome.
But if I fail again and again I can give up. This mostly happen if the game relies too heavily on stats, and I can’t manage to obtain a good ending because I focus more on the story and RP.
Now, this is for text based games. When I play visual novels, usually I either do only one route, same as in text based, or I 100% the game.