You are, as you say, character heavy. I’d certainly drop the level 3. With the spare points you could get more bull centaurs for a second unit, or a hellcannon. Or you could drop the numbers on one of the infernal guard unit and get a destroyer. There are loads of things you could put on 100x150mm bases to proxy for either at the moment. But you’re not going to have enough pd for two lord level wizards to be effective.
In general, the equipment set-up is good. I’d consider getting the crown of command for your level 4, putting him in one block and the bsb in another for two stubborn blocks - very hard to shift! I don’t know about the taar’uk as I don’t use one much, but he could get the (other?) tricksters shard - the one that forces re-rolls of ward saves - for a decent assassin. Maybe gleaming pennant on your bull centaurs?
With the Taruk and the big block of Bulls I like the +1 move banner and its cheap. I dont like the 2nd SP I agree that its too much caster and not enough dice I would bump up the deamonsmith to lv 2 and take lore of metal. spend the extra points you saved from that some place else. More Bulls, Artillery, couple of khans somthing like that.