I haven't yet made or worked on any statsim game, but from player's perspective can give you some insights that may be useful:
As already stated above, LLTQ did very well in that regard with it bad+alternative endings, but the best implementation of this I think I've seen in
Gaokao100days. Your ending combination (the college you get into and the girl you get) depends on your performance but you WILL get some ending regardless of how good of bad you are. This not only gives player freedom, but also encourages replayability.
IIRC there are also some special options - like for doing certain story choice-based events chains in a certain way, or having some secondary stats such as PE high, or walking in the park every day, etc. So essentially if you delve deep into some side things your stats will matter less (but still will matter). Bad ends, too.
Overall, my points is - a good statsim should not lock the player into hardcore "mathloops" where they will be forced to "minmax or fail". Instead, offer them a good story with a variety of endings.
I may be wrong, but I think most people play statsim for freedom of developing their character themselves and to see how story/ending plays out based on their choices in the game. This way, game is never too easy or too hard - slack off? well, get "so-so" ending. Want more? Well you need to plan better next time!
But, still - never lock stats too hard - because if to succeed in your game, player would need to minmax, it would basically kill all the freedom and posible alternative opotions. When it's like: "go to school event or skip and meet the mystery character in park" it is viable, but when its like: "meet the mystery char in the park or spend day leveling generic stat to not fall behind"(=if in your game options to level up stats are scarce enough for player to be seriously considering whether to see interesting event or level a generic stat ) - it is bad design, IMO.
Because, as a player, if I wanted to just do math, I'd just do my homework or something. Statsims are RPG games first and foremost. And freedom is the key in that.
And adding multiple endings in a statsim is much easier than in regular branching VN. Because, instead of dependency chains and expansive events sequences, you can just add fixed endings based on final stats + whether or not player have seen certain events/made certain choices in them. And adjust them easily, too.
P.S. writing that last one made me actually think about my own current project, maybe I should scrap my hell of a labyrinthine branching and make a statsim instead...
