mac mini (2018) displays not waking properly after boot or sleep

Mac mini (2018) has always had strange ways of displaying on the monitors when booting or waking from sleep: the monitors always flickered on and off a few times, but showed ok in the end. This was with each monitor connected to a USB-C port.

As I needed a USB-C port that the (main) monitor was using, I switched to using a HDMI cable for the main monitor, and free up one USB-C. Well, since then either both monitors stay dark after boot, or just one shows, the external one connected via USB-C, as the main screen, with menu bar and dock.

Only after switching off and on the main (dark) monitor the machine detects both monitors, and the situation is corrected. Any idea how to fix this?

Config: mac mini 2018, macOS 12.5, DELL U2715H as main (HDMI), DELL U2713HM ext (USB-C -> Displayport),

Both "Default for display" resolution. "Displays have separate spaces" is OFF.