Re: XP, just stop giving XP for killing things. Make all leveling milestone/story-based.
Well, first, I'm not talking about "a single round of combat." A deadlier combat would likely have multiple foes, or at least one foe with lots of actions, legendary and lair actions, and all that sort of stuff, and monsters moving around instead of going toe-to-toe, causing it to last long enough that the casters will have to use both higher-level slots and cantrips. Also, if you have both combat and non-combat encounters (obstacles, hazardous conditions, social encounters, etc.) then that also helps to deplete resources.
I do agree with that. They shouldn't have used that at all. They should probably also completely redo the CR system to both take other things into consideration (most effects that cause conditions, even conditions like incapacitated, or that reduce hp/stat maximums, don't actually affect CR) and make it so CR 1 = 1 first level character, or something like that, rather than equal a party of four.