Oh well. One project failed.
Just noticed that my crate for DJing on the Windows partition is 50GB, mostly because there's lots of FLAC.
The advantage of FLAC compared to ID3v2 is that you have Vorbis Comments, basically free-form key-value pairs. That's also the disadvantage.
There's no standard on tag names, capitalisation etc. People can't even agree on how to store multiple artists for one song. The recommendation says to use multiple ARTIST tags, but real-world users don't do that.