I'm of the opinion that a lot of problems could be solved if the spell DC was based at least partly on on the level of the spell slot used. You'd still have to adjust the CC spells where upcasting gets to target an additional creature, but generally speaking a low level CC spell should have almost no chance of succeeding against a high level boss monster, you need to use a high level spell/spell slot if you want a decent chance at CC the boss.
Failing that having a Legendary Actions that can break CC effects works decently.
				
			Failing that having a Legendary Actions that can break CC effects works decently.
 
				 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		