@nicomiguelino @ealmonte32
Thank you for your patience. I have now made sure to use the correct image (2022-09-22). The only mostly-working installation method via bash/curl is still producing the same issue: It installs fine (as it did before), the web interface is reachable, file upload works and so on - but the display mostly flickers white or shows partial images, also it appears to take a long time to switch between pictures.
I did not find time to grab a working RPI4 with an older Rev. firmware and dist-upgrade the whole thing directly on the screenly sd card to see if that makes it compatible with the latest(?) RPI4 v1.5 firmware boards.
As it stands, the other methods already fail as they can’t pass the RPI boot rom, stating the board needs newer firmware to work than what is used on the images. And the manual installation method - see above. I also tried downgrading the firmware of the RPI but without success.
I’ll let you know if I find any cause and/or solution for this weird behaviour - and of course I’d be happy for any further pointers how to fix the issue 
Kind regards