Is there a good Mount & Blade Warband mod that doesn't break the game one way or other? I don't really want to deal with one-hit-KO swords and omnipotent bandits thank you very much.
There's Blood&Steel which... tbh might not fit your criteria
exactly, but in a sense that it gives the player the same tools as the AI, more or less. So this mod is kinda like 16th century Calradia (but again, not quite) with a fair mix of melee & gunpowder units. The catch is the units are silly-powerful. Battles are explosive, fast and most importantly, FUN. Enslavement system is reworked a bit, with max prisoner capacity bigger by default and amount of captured people calculated automatically after battle (no more running around clubbing people, hoping to get them high-yield prisoners) and it more or less works. Regular bandits and gameplay is, iirc, more or less akin to vanilla though, so no looter deathstacks right in your face from the beginning. A sort of a cherry on top is hordes of undead that roam the lands. Big, strong hordes. You engage them for fun and profit.
TBH I never played it to a point when I'd experience kingdom vs. kingdom warfare so I have no idea how broken those battles get with new troops. I think I remember something about lords having some ridiculous-sized armies as the time goes on, I mean iirc I saw undead having 1000 troops. But given the carnage your troops (and you!) are able to dish out and also take, I'd say it makes for some fun times. Again, fast and fun. Definitely the complete opposite from my experience with, say, Pendor.
Theeeen again it's been... year(s) since I last played it? No idea how the mod changed in the meantime.