In my opinion, adding more mobs to the area would be a bad call, because then it means you'll just make it so that to fight it you need the best armour and best weaponsI'm not changing how taint spreads, it does that fine on its own. I want to actually instill fear when someone sees a taint biome creeping up on them, make it dangerous to try and fight it. Purifying it means going in and placing blooms, but that would mean having to fight everything that thrives there.
I'm thinking adding more stationary monsters, so stuff that comes up and is damaging (Like how I think tanticles are like). I'm picturing a taint hydra, so a big tendril that tries to hit you if you come close to it, but the caveat about this one that you can't avoid is it will spawn smaller weaker versions of itself towards the player, so that if you let a taint biome come too close to you, you'll get these tendrils that will pop up out of the ground nearby you, and eventually in the middle of your base if you don't go and deal with the main one.
EDIT: And if you can't manage to deal with the larger tendril, having to go and periodically trim back the tendra spawns (Tenticle + hydra) to keep them from popping up in the middle of your base.
I think it's important that it targets players, so that if a taint biome is chunk loaded for some silly reason, it won't have 1001 of these taint tendrils about