How much EM is too much EM?

For this team. I had Mavuika/Furina with Key/Citlali C2R1/Nahida. Basically Archon team.

Obviously not the most optimal team but this made me wonder if I should swap the EM piece for Atk?

In Mavuika/Bennett/Citlali/Xilonen. I have around 478 EM How much damage increase is it?

Also about Citlali is it worth it to build Citlali with ER/Cryo/Crit rate when she is C2? Or ER/EM/EM all the way.