i don’t know, sorry.

sadly, yes. the custom rom scene is really dead these days. there are some smaller roms like evolution x, paranoid android, blissos, derpfest (probably a few more) but i’m not sure if using them is a very good idea. honestly haven’t seen much useful feature from those since android 9 days… i just use lineageos btw, but usually an older version (<=android 11 so i can use gravitybox, which gives those customization features)

if you can bootloader unlock them, it is possible to install a treble GSI rom, since all android devices released with android 8 oreo and later features treble. try searching phh-treble, these roms are built to work on as many devices as possible and some features may not work properly.

sorry, i have no idea about this. maybe you can lock the phone down to the bare minimum by hiding/disabling all the apps except the essential stuff? lineageos stock is pretty much already that, just need to remove the web browser with adb pm uninstall i believe.