Not every encounter can be a grand engagement.
I would argue not everything needs to be an explicit encounter.
But its also the fact that its a little strange to still be inundating a party of level 20s with useless tedium encounters.
PCs at that level are meant to be multiversal heroes. Sure, not every adventure for them needs to be an endless string of epic grand battles, but at the same time you should be reconsidering what you're counting as a normal encounter; fighting a handful of mooks or trudging through the same mundane forests you had them do at level 3 is a waste of session time at this level.
They don't have to be all grand battles of mass warfare to still work towards keeping the players sufficiently occupied; mass warfare is just the easiest way to hit the marks. Solo skill challenges in tandem with group challenges are the next best bet, and then you have your standard puzzles, heists, social encounters, etc that can all be scaled up and configured to keep the group occupied.