Yeah that OSMC box is just running Debian with their stuff coming from its own package repo. You can get a root shell. I realise I could have built something myself (and have in the past) but it's absolutely worth the money to me to get everything in a tiny package and working perfectly from day one.

I wouldn't recommend Kodi for streaming, it kinda works but the experience isn't great. I use it exclusively for playing stuff from my server full of legally acquired public domain videos (ahem).

I do watch YouTube videos on it, but I use TubeArchivist (basically a fancy wrapper for yt-dlp) to pull them onto the server first, and a script to organise them into nicely-named directories.