Here is another issue I have run into (and could use thoughts on): while the characters have these blizzards of abilities, spells, actions, reactions. bonus actions, etc.---so do the freaking monsters and NPCs! As a DM, I have found it hard to hold all of this stuff in my head while creating encounters and running combats. Not only am I keeping what the characters can do top of mind, but I have to manage what their opponents can do, too. Any thoughts on how to better manage that?
At the end of my last campaign, the PCs were 6th level. As the big show stopper event, I basically made mirror images of each character, complete with weapons, spells, abilities, etc. and said "have at it" and them run the combat against themselves. It was...insane. Just insane.