I used Linux Mint on a Lenovo Thinkcentre micro-PC for a while, at 2x scaling on a 1080p TV. Now I've changed over to KDE Neon (because the KDE UI is more flexible) at around 2.5x scaling, and it's a bit more comfortable. The main input device is a Logitech wireless keyboard with a touchpad. I mostly play videos through SMPlayer/VLC, so it doesn't need anything fancy.
A dedicated 10-foot UI would maybe work better, but as a low-cost low-effort solution, this works great. I got the PC for free (somebody was throwing it out at work) and the keyboard plus a DP to HDMI cable was $35 total. It plays 1080p x265 just fine.
Have you considered any of the ELEC releases? These are Kodi on minimal Linux with a diverse plugin ecosystem, with docker support. I am using LibreELEC. There is a live option in the installer to give a whirl.
I have looked at them, but not seriously. Maybe it's time to give it a whirl, thanks :-)
I think the GP uses the MacMini as a "hack" to be able to back up the NAS cheaply with Backblaze's unlimited personal desktop backup offering (there's no official Linux client, imho):
"Get peace of mind knowing your files are backed up securely in the cloud. Backup your Mac or PC just $5/month."
And use their GUI program on MacOS and sync that remote NAS storage folder.
I'm not sure if Linux would make for a play-for-play better home theatre OS than MacOS, but it would certainly be both better than Windows, and better value than a Mac.