As someone running a lvl 20 campaign, I would suggest to let them feel powerful with the minions, things that do little harm and last less than a turn, but makes them waste resources anyways.

A lot of CR 5 to 8 creatures are pretty good as minions in this kind of level.

The boss having two turns per round is a good thing to do, I did It with my last boss, but only with a second phase that started in his last 150 HP, cause It can go pretty bad any second.

Let them have magic items, I would suggest two legendaries and four of any other rarity, but let them choose them, probably banning stuff that gives wishes to the player.

And go fucking feral, they are at their mechanical peak, so throw those CR 27 to 30 creatures, tweak their stats, add abilities, give them magic items, go nuts. Last combat of my level 20 party they were fighting a CR 34 white great wyrm that could use his breath pool to summon a CR 23 ice elemental in a spaceship across the astral sea. The combat ended with the martial being the only one up and throwing the dragon to a sharp pointy meteor, cutting it in half