I’m currently building a similar device, which uses miraclecast for screen sharing from a phone or laptop and jellyfin in firefox kiosk mode controlled via the jellyfin app as a idle screen. Also sending media links via kde connect is possible. I still want to implement hypnotix for tv and some kind of intigration with the streaming services of the german public brodcasters. I haven’t thought about dlna but I like the idea.
I’m also using Debian as a base but I’m using LXQT with Wayland insted if Plasma. I tested everythin on a rasberry pi 4b but it’s struggeling. I want to move everything to a rock 5c.
I dont know how you flashed the usb, but it seems like the installer is damaged. Try redownloading the iso, check the file hash, flash the usb drive with balena etcher and reinstall.
Did you change the partition layout in the installer?