I have been chasing a similar problem for months. I also have a late 2018 Mac mini (running Sonoma 14.7.2; am reluctant to upgrade to Sequoia). Unfortunately my display (LG UltraFine, TB port) does not have any display controls to nudge.
Weirdly, what is now working (*) for me is:
- I set the Energy Saver settings to: Prevent Automatic Sleeping When the Display is Off (YES); Enable Power Nap (NO); Wake for Network Access (NO); Start Up Automatically After a Power Failure (YES).
- I no longer put the system to sleep; instead, before leaving, I choose Lock Screen (instead of Sleep).
- There might have been something else; I'll have to think about it.
(*) Since adopting those changes (roughly 4 - 6 weeks), I've had one instance where the display stayed black and blindly typing in the password did not work. In that case, I got the display back by using my old work around (connecting a second display via HDMI and after rebooting the system, the LG UltraFine magically comes back). Oh! And as a bonus, with the changes, the display now comes back after a single key press. Before, I sometimes would have to press a key several times before the display woke back up. (And of course, most times it didn't.)
There was an old thread (in here or maybe MacOS, I forget now) that suggested those changes, I guess implying that there was an issue with Sleep and the display or the port or the late 2018 mini.
