I don't disagree that the BBEG is going to have defenses and such, I am just saying that in my experience a half dozen players with a broad array of high level PC abilities are generally pretty good at finding solutions when it's just me sitting on the other side of the screen. I generally do not like fiat countering as a GM (i.e. just inventing things to stymie the PCs at every turn). Rather, I'll decide villain X has defenses Y in place, and then leave it up to the PCs about how to deal with those things. Sometimes they draw the villain out. Sometimes they summon an army of umber hulks to dig a tunnel into the basement. In any case, it is more common for characters of that power level to choose the time and place of battle, in my experience, which has the effect of allowing them to nova. That in turn means I feel like I need to make sure BBEG's are extra powerful to make them an interesting challenge, but that can lead to "unfair" feeling situations if things go against the PCs.
So I guess what I am asking is in high level games, what are some tools for making sure the PCs have had a few resource draining encounters prior to those set piece showdowns? I like letting them feel smart and strong, and I like challenging set piece battles, and I don't especially like grindy fights meant to chip away at their hit points and spell slots, and all those things feel kind of opposed.
There's not really anything wrong with them doing that sort of thing from time to time (trivializing the showdown). They probably enjoy it.
I think your approach of establishing the enemy's capabilities without retconning is good.
That said, are you sure some of this experience isn't coming from 3.x? I'm not aware of anything in 5e that allows you to summon an army of umber hulks. High level 3.x is its own thing and doesn't necessarily translate to 5e.
As for challenging them, keep in mind that you can play pretty fast and loose with tier 4 in 5e. PCs can typically handle challenges that seem unfair on paper.
For example, if the PCs lure the enemy out of their lair, then have the BBEG bring all of those set piece encounters with him. Even if they can't win, they can probably teleport away or escape by other means, and now they've learned that just because they try to dictate the terms doesn't mean the BBEG will necessarily respect those terms. Just like if they try scry-teleport-kill and find themselves in the middle of a death trap.
Sometimes the right clever approach should trivialize the encounter. But if the enemy is more clever, it might make it harder. That's the risk you take. If they approach the game as CaW then they should expect to be engaged the same in turn. If they don't want a fair fight then they shouldn't expect one.
As for bringing the set piece to them, just have it be the function of some evil artifact or the gift of a dark deity. At that tier you can pretty much get away with whatever explanation you want. When the PCs try to engage, the magic brings them to your set piece battle. Or the bad guys used mirage arcane to make the set piece battleground look like something harmless. Or whatever.
Just like the PCs have less limitations on them at that level, so does the DM. Obviously, you don't want to abuse that but you shouldn't hesitate to use it.
Of course you still get the +3 damage, so not "no effect"
