ydls VS tag

Agreeing with this. Unless you have very specific performance codec or packaging needs i think external ffmpeg process etc will work fine. In my experience with transcoding the cpu/memory/etc overhead of an external process is usually dwarfed by the actual transcoding workload. Also the extra process overhead is not typically a problem as you can't really do that many concurrent transcodings on one host anyway. If you want some inspiration you can have a look at https://github.com/wader/ydls

Most music managers on Linux seem to be rapidly disappearing, and so I'm having to say goodbye to my beloved Banshee. Since I don't really love my other options and in general people seem to not like locally saved (and bought) music with a music manager anymore, I'm playing it safe and making my own. I've chosen most of my libraries and done some small tests, but a hurdle that remains is metadata. Sadly, beyond MP3's ID3 tag system, there's custom stuff for a bunch of formats/containers, including ogg/opus, flac, and m4a. https://github.com/dhowden/tag seems to be my safest bet for reading. It has very wide support and an active community. Writing seems to be an ongoing struggle however, and I can't seem to find anything that really works. It would be weird that you can't fix a typo in an album name, update a genre or split up between the album artist and track artist. Does anyone have any suggestions?

Ok think i would either use some metadata library like https://github.com/dhowden/tag or ffprobe with JSON output to read metadata and then use cli ffmpeg to transcode and write metadata. Maybe later if you feel it does not give you enough control maybe concider using some cgo libavformat/libavcodec wrapper.