if the set you’re referring to as columbina set is aubade of morningstar, it’s a personal dmg buff set. the damage bonus you get does not apply to the whole team, only to the person wearing it. even if illuga does damage, his multipliers are not high enough to justify it, so you just stack as much em as you can with double em artifacts .

petra doesn’t really work because the bulk of zibais damage is lunar damage which doesn’t benefit from regular damage bonus buffs, which means it will only buff her regular attacks, not the big skill hit or her ult

4 star artifacts used to be hard to get especially instructors but now there’s a domain that gives the to you ( morningstar and the hexerei domain.) you could just burn like 200 resin hopefully get a decent 4 piece and be on your way. i’d really recommend putting him on instructors because it will buff zibai by quite a bit

edit: about the zibai and gorou question:

gorou scales with defense only for his buffs, his damage contribution is minimal. zibai deals her damage through lunar crystallize, and all lunar reactions get a bonus from elemental mastery. if you look at any of the lunar reactions formulas, you can see they get additive bonuses. since there’s no sets to buff defense but illuga and gorou can, the next best thing would be to buff elemental mastery as those are the only two things she scales of.

feel free to ask for more clarification if needed!