I know it’s bad form to suggest using other software that handles the same functionality instead of suggesting a fix, but it looks like sddm doesn’t have the functionality to change displays at the time of writing.
GDM seems to have a workaround
But it looks like every display manager chooses whichever display based on arbitrary criteria.
Don’t use hacky unlocks if there’s an official way. The best case scenario is it becomes a headache and isn’t reliable; ghe worst case is that it bricks your phone or installs malware in the bootloader
All I could find looking for custom ROMs for your phone was XDA users shrugging their shoulders and unverified downloads from very shady websites/githubs. I’d suggest getting the most out of this phone you can before selling it and getting one you know works with the OS you want