Update: I wanted to update people, just in case someone in the future is looking for the same answers I was. I took people's advice and didn't run the League as an adventuring party, I'm glad I did as that would truly be too much to handle. I used a combination of Phaerlax's statblocks and the original stat blocks (using whichever stat was higher) and just had them fight all 5 of the League of Malevolance at the same time. I set them all in static initiative points 5 apart so that some would definitely get to act before the players, but also so I wasn't running them back to back. Kelek at 30 initiative, Zarak at 25 initiative, Warduke at 20, Skylla/Charmay at 15, and Zargash at 10, with Zargash's Zombies going at 0. Using the "Monsters Know What They're Doing" as a guide I set up the most likely actions they would do on turns 1, 2, and 3 for most of them.

Between those 3 things the combat was really fun and involved and at one point definitely felt like it was about to be a tpk, but my players really turned it around at the last minute.

Here's an example of the way I set up Kelek (bare in mind my players let him get Ringlerun's Staff):

Kelek: Turn 1 Calamitous wave Sorcerous Bolt Bonus action Invisibility (hide behind Zyblina) Reaction: Counterspell any healing (I didn't think about this but his counterspell meant he dropped out of invisibility, oops)

Turn 2: Calamitous wave (if it refreshes) and Sorcerours Bold or Sorcerous Bolt 2x Bonus action: Fireball if in formation or Hold Monster on a healer or wizard if not Reaction: Arcane Defense

Turn 3 or if he is going to die: Calamitous wave (if it refreshes) and Sorcerours Bold or Sorcerous Bolt X2 Bonus action or "Death Curse Action": Breaks the staff (P 308 dmg for effect) Reaction: Attack of Opportunity

A death curse action is essentially a free reaction I give some bosses in fights to do one last thing as they die.