Hope I'm right here with this, sorry if the question is too specific for this community.
Our long-term D&D campaign has reached PC level 17. Now's the time for implementing mechanics that fasten up combat (we all don't like how long it takes), as well as addressing spells that the GM (me) fears to be game-breaking (mostly 9th level spells, full list below).
Sooo... I'd be thankful for any helpful suggestions. My current ideas for the combat aspect include shock damage, Instinct and Morale checks from Worlds Without Number, but those probably don't suffice yet. And I'm having trouble conveying to the spellcasters that toning down some spells is necessary in order to maintain a functional game that doesn't tear apart hours of preparation every session. Feel free to point out any potential counter-arguments to my view on that as well.
I'm really determined to keep this game running. Thank you all so much in advance!
List of spells that require revision or more exact definition (in my opinion):
Wish, Meteor Swarm, Invulnerability, Gate, True Polymorph, Leomund's Tiny Hut, Rope Trick
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.
I like your approach to speed up combat, although I inevitably will have to look up stuff and I would understand if my players would view it as unfair if only they have to skip their turn while this wouldn't apply to NPCs.
We have repeatedly debated the matter of spells for almost a year while never quite reaching a consensus. I understand your opinion on winning D&D, I just think that the game will not be able to develop any depth anymore if the preparation can't be relied on, being replaced by an inflation of poorly thought through plot hooks made up on the spot.
I mean--it's your game. Your table are the only ones who have to be happy, and you certainly know them better than a bunch of internet strangers, so take all this stuff with a grain of salt.
For what it's worth, I've personally made the mistake of trying to rein in the insanity for high level characters or just broken splats, and the result was just discovering a new suite of toys for the PCs to abuse--or worse, outright resentment. Take care that you don't mistake for assent or consensus what is in reality just player unwillingness to openly voice their unhappiness. After all, most folks will rather play watered down D&D than no D&D. And if nothing else, developing the ability to write plot hooks that can survive Plane Shift, threats that overcome Permanent Contingency (no longer a thing in 5e), and Wish outcomes that feel both amazing and terrible is how excellent DMs are made. Do what's best for your group, but you're also part of that group.