Just this morning I was thinking what to do with the list of instances you posted on Who is missing? Who should we invite?. 
I think it is a very good idea! You can start it, we turn it into wiki, and then add it to the Knowledge base index.
But why just micro/mezzo blogging only? We could open it to other fediversal platforms as well, PeerTube, etc.
Maybe in separate sections, yes, and then people can choose based on their priorities.
Ideally, at least the admins / members of the music-focused instances should be aware that this page exists, they appear there, and they can keep the information updated if they wish. We can also have a disclaimer at the top encouraging people to improve and update the page if they see anything outdated or missing.
EDIT: forgot to mention that I would be happy to reach out to admins of instances mentioned in the first version of this wiki page.