In reply to all:
Thanks for the many suggestions, that's a good selection apps to audition.Audify and Media Monkey both have the same problem as YT Music: albums listed twice (though not all albums, in the case of Media Monkey, which is even weirder) with the tracks split between the two listings. No idea why this might be happening but it makes them unusable for me. It also took Media Monkey a lot longer to scan my music library than the other candidates.
I am familiar with VLC on my Mac, but I don't like it very much - the UI could definitely qualify as 'venerable'. I only use it for stuff that Quicktime Player baulks at. The Android version seemed to have a tendency to re-scan my music library at the slightest provocation, which was tedious.
Rocket Player seems to take an age to fire up. It scanned my music library OK but doesn't seem to have a clean exit function (compared to Musicolet, for example, which is just tap the back arrow twice and it exits).
At the moment Musicolet is looking like the preferred option: very straightforward UI, and it scanned my music library quickly and without problems. It does seem to be integrated with Android Auto: it's listed in the settings, but I haven't tried it in the car yet. I will keep Poweramp as a backup option for now.
(One minor annoyance with Android Auto is that Google Assistant doesn't seem to be able play music from my library - it insists on using a streaming service 🙁)