Thanks for the MakeMKV suggestion, I'll look into it.

VLC isn't an option at the moment, because we've hooked up a 60€ player to her television.

Something with a remote with physical buttons that boots up in two seconds flat, never updates, always works the same and needs zero network connections.

A pretty high bar to beat.

You have described a Raspi media center with a remote and the wifi turned off. It may take a couple seconds longer to boot, but you get back VLC et al in exchange, and there are many good, free options to use it as an interface for your NAS.

What a time to be alive

I have one lying around anyway, so maybe that's another viable path. I'll take a look at how appliance-like I could make such a Raspi, in terms of UX.