I read through the entire thread, very illuminating.
What your reviews lack for my use case is mentioning the library size, scan times and if you only test playing local files or also using the player as a client for a server (if the player supports that). You also don't mention (or I've failed to notice it) lyrics support, which is a huge factor for me.
Symfonium is the only app I've found to be "working" for my library (814k tracks, 21.4TB). By using Jellyfin as the server and using its API, I can have a local database of my entire library in Symfonium, with working albumartist, multi value fields, properly displayed album art (none of that cropping everything to fit 1:1 nonsense), artist art and lyrics support. (Well, so far only embedded lyrics. Support for external .lrc files -also over the API- is already merged into the Jellyfin code and will become available in the next version.)
What I love about Symfonium is the seamless mixing of sources. You can have multiple different music servers (Plex, Emby, Jellyfin, Subsonic / OpenSubsonic, Kodi and local files) and use all of them at the same time in the same database in Symfonium. Or if you only want to see the local files or a specific server, there are quick and easy filters for only displaying what you want/need.
That's a unique approach I haven't seen in any other app and the local database is performant enough not to choke on my library size. When I search for an album in plex or jellyfin directly it often takes 20+ seconds for the GUI to respond. In Symfonium it takes a few seconds at the worst. It's the only app where looking for music on my phone is not painful.
The developer (Tolriq) is also very active and usually responds within a day or 2. He's already added around 6 different features I've asked for over the last few months.
If you haven't tested it yet, I suggest that you try it. There's a free trial period of a week I think.