This is a situation that I expect to get much better under the Apple silicon Macs. Under the current situation, Apple is writing drivers for hardware they did not design, and switching between discrete and integrated GPUs has been a flaky issue going all the way back to G4 PowerBooks. A lot of this complexity is going to just evaporate.

Incidentally, I was also using a Thunderbolt Display with a 2016 usb-c MacBook for about three years, it was similarly flakey. Switched to an LG 5k (found an inexpensive refurb) a while back and 95% of the problems went away.