Earlier quoted context omitted.
I've been searching for one hour, and the only truly safe thing is an HTPC linked to an OS-free commercial display like these large VAs from ViewSonic: https://www.viewsonic.com/global/products/commercial-display... (goes from 45" to 100").
What software would you install on the HTPC? User will typically want to have a remote control and have applications like youtube.
So probably a custom lightweight distro with mpv outputting to DRM directly and a game controller to navigate some file explorer ? Maybe try to script around some speech-to-text LLM to be able to say "Play me X", heh.
Or just control it via ssh because that's actually simpler for me.