Glad you liked both girls! Yeah, MetaSarah is something I really want to do, I've been slowly setting it up but gotta get that payoff eventually.
Siofra is 3 distinct phases, with a 0.5 if you count the summon during her first phase. She's definitely intended to be hard - I've referred to her as the 'first real boss' because everything previous was designed to make it easy or at least pretty doable, whereas Siofra (even after her nerfs LOL) can still absolutely ruin your run in phase 3. You were her first 'boss fight' so to speak, and she realized she was having fun LOL
Not every future boss will be the same, but even the temple boss had certain things happen at various HP percentages, the wolf had its super Mark to dogpile the target, etc. Future bosses will have their own mechanics, like minions you don't want to target, near-invincible bosses that take damage when you kill minions, etc. Ideally, it'll be more than just HP bars. But I can definitely say that most won't be as difficult as Siofra.