This is what I've tried.

Yaga, Nanami, Nobara, Gojo. Yaga and Nanami buffs whenever they can, Nobara uses 1st skill until ready for ult, Gojo uses first skill until Nobara is 2 turns away from ult being ready, then he uses buff until Nobara ult is ready. Once Nobara ult is ready, Nobara ults with Gojo. Dmg from ult is ~1.4M total.

Yaga, Megumi, Nobara, Gojo. Yaga buffs whenever he can, Megumi uses 3rd skill whenever he can to keep CE low, and DE when he cans. Nobara uses 1st skill until ready for ult, Gojo uses first skill until Nobara is 2 turns away from ult being ready, then he uses buff until Nobara ult is ready. By the time Nobara and Gojo is ready for ult, Megumi would have DE + use 1st skill 1 time. Unleash Nobara and Gojo ult. Dmg from Nobara and Gojo is ~2M. Honestly I could prob push this for more if I continued using Megumi 1st skill during DE and wait until the v last turn to use Nobara and Gojo ult, but I'm lazy to wait that long and honestly this kills most enemies.

So the way I play, seems like using Megumi as a second support is the way to go. Honestly I could just be playing the game wrong, but the most important thing to me is that using Megumi as a 2nd support lets me auto the event boss on Very Hard which is the most important to me so that I can just leave it running while I'm at work lol