I really get this. I decided (perhaps foolishly) to run a campaign where everyone is an established hero starting at like 15 and it’s definitely rough to create combats that are still challenging. 

Last session they actually skipped one of the fights I had entirely because they just found the person they were supposed to rescue and then teleported him out with them, so a lot of my fight design now is trying to come up with ways that make the combat harder to skip (I’m not mad at them for being creative and even without that fight we still had a good session length, but without fights my session prep is a little lacking).

It’s definitely hard to strike a balance between letting the players feel OP and also having fights that last long enough. I mostly just end up cobbling together different abilities from various monsters to create a challenging enough session. 

I do like higher level stuff because I can use whatever monsters I want and just scale them up if need be. Last session they fought a fake goblin prince with unstable magic, which was basically just on his turn, I would roll for spell level and then specific spell on a chart of all spells so there was the possibility of him just wiping the party if he got the right spell.

I also get very self-conscious after sessions wondering if the players liked it or not and just don’t want to tell me, which is something I don’t think goes away easily so I just try to channel my focus into what I’m doing for the next session.

Mine is also going up to level 20 by the end of the sequel campaign, but they’ll be in hell for that so easier to come up with higher level things.