Now I got Plex behaving, I think. Removed and re-added Library, and under Advanced, there’s a separate/additional dropdown for Genre, where I set it to only use Embedded Tags. If I left it at Embedded Tags instead of Plex Music this time. (Still scanning.) Somehow selecting Prefer local metadata is not enough, I also have to select this one … 
So now I have two good options running, AirSonic and Plex.
Plex mediaserver seems to use less resources, though. Running it on a 512 MB RAM KVM with just HDD and no killer CPU. AirSonic runs on a Ryzen VPS with 1 GB RAM and Docker. Seems Java and such uses resources …
Being able to use Clementine to play is a big plus, though. 
… I now it’s not the most Apple friendly crowd around here, but any hints as to finding the best subsonic client for iOS/iPadOS?
How would this gonic compare to AirSonic? Lightweight sounds good. Works with the SubSonic clients I assume?