Another point to add is that the definition of upf is extremely broad and vague. And there isn’t much evidence that this whole category is harmful because of how broad it is. But hey, maybe it will still result in healthier meals overall nevertheless.

MP3 is fine if the bitrate is high enough and it was encoded with a semi-modern encoder. I still prefer FLAC so I can transcode it to whatever lossy codec I need without additional losses (usually Opus but there may be better codecs in the future).

Maybe Navidrome’s jukebox mode, although I suspect it’s a slightly different idea (and it was a bit buggy last time I tried it).

Many don’t run into dir structure issues because they have already organised their libraries with beets or an equivalent. I don’t personally have any issues with playing albums either. Some of the issues you are talking about seem to be client side, are you only testing the web interfaces?

Dsub2000 and Tempo are active FOSS alternatives.