The PCs won D&D. Let them enjoy it. PCs who cast 9th level spells break reality, and breaking reality is okay. This is the thing they played to win, so let them enjoy the fruits of their effort. They're going to tear apart your prep, and you need to get comfortable with that. Expect to improvize.

There are still challenges you can throw at them, but they have many tools at their disposal. Your job is to give them opportunities to use those tools--not remove them. Anything they can do, an NPC can do back.

Combat speed and pace is a different animal, and the best advice I can give is to forbid bookkeeping on turn, at least for level 17 experienced players: if you have to look something up when it's your turn, you get skipped. Know what your stuff does if you're going to do it, and use the other turns to decide on your action. If you need to read something from the PHB on your turn, you need to already be on that page ready to go when you come up.

Ultimately, you're the DM, so you can say what is or isn't allowed and what's a reasonable turn length. But you still have to honor the social contract. If a player made a wizard and played it for 17 levels because he wanted Wish, the time to adjust Wish for your table was 17 levels ago--not the session he's finally got it.

The biggest challenge during Tier 4 is still resource attrition. Let them use their big spells, but don't let them rest. The best challenge you can give them at this point is to make a multi-session-spanning dungeon-like structure.

An example from my previous campaign: heroes needed to get to the lowest level of Hell, but they needed to transit through every one of them in process. Enemies were everywhere, and places for rest were virtually nonexisting. I think they had like 1 long rest in four months of play during T4, and it actually was hard for them.

Hope I’m right here with this, sorry if the question is too specific for this community.

I think the community is small enough to not get split between 4 sub communities. I don't play D&D but can't help much ?

Have you tried to evolve toward a more ? Strategic management game ? I assume that at this level they aren't anymore the one crawling dungeon but the one hiring NPC to to do. That they own a whole continent if not a whole plane, and even though they could use the nuclear option the political/diplomatic consequences might be to big. (think about real world, and how nuclear power still loose conventional war because of the political price of launching a bunch of nuke and turn a hostile country into a radioactive wasteland) Now is it what your PC want to do ? That's another issue