On the other hand, level up too much, and you'll end up like that level 60-something Alice that solo'd Eientei by simply having too much goddamn defense.
That's the thing; because of the way defense works, if you just quarter the level-up rates then everyone will start taking 0 damage from absolutely everything; well, err, apart from the fact that non-Wriggle peeps have terrible defensive stats. Wriggle, though, would be pretty much invincible.
Their individual growths are as they are for balance reasons and tbh, probably don't need tweaking. I don't think it's such a bad thing that I'll be overlevelling to beat the bosses because the conditions necessitate it. How much overlevelled I will be is what keeps it challenging.
So Wriggle being used by basically no one (And she really isn't worth using compared to others), Cirno being a piece of paper with fast SPD/PAR debuffs only (And weak PAR, at that), Rumia not even being VIABLE for using as she's zetta slow, pitiful HP, weak DEF and only decent MND, Moonlight Ray does what damage that everyone else should do on non-nukey multihit attacks, her MND-ignore is pretty damn weak and there's Kaguya instead, Demarcation is nice but it isn't anything to redeem her at ALL... that's balanced?
Not saying you have to tweak stats or anything, but these characters (minus Mystia) really aren't balanced. Cirno is useful for awhile, I do admit that. But Rumia, she's throwing Silent Selenes around for a floor or two, and then she falls behind everyone else
so hard and there's no way she could possibly be usable at all when you've got other choices.