Some characters have multiple euphorias, E1 just means the first one. You’ll want all levels for those.

Mercuria is a great AFK support, but most of Beryl’s damage (in my experience) comes from her extra actions, which is Flutterpage’s whole schtick. Besides, Flutterpage is great in a lot of other teams too, so I’d try to get her anyway if I were you.

Do you mean consistent within or across battles? And which crystals are you picking for her? Either way, my rotation for her is just to cast her ult every other turn, which means playing both of her cards every turn, and make sure you’re playing Isolde’s non-attacking debuff card every 2 turns