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
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.