Everyone else is telling you to go update musicbrainz, and that's a sensible course of action for the first two bullets, but the much easier path is to import all these files with whatever metadata they have.
There is no canonical metadata for a fan recording of a concert or DIY CD-R, so you lose out on nothing by importing the files as they are today.
Once you're over the hump of the first import, beets is a fabulous tool for ingesting new music. It's well worth it.
This is the answer I arrived at myself a few hours into importing my own music library. Cleaning up metadata for commercial releases is a nice feature beets offers, but the real value (IMO) of beets is it's a powerful toolbox for managing a large music library, and that's true even if you were to eschew musicbrainz integration for all your music.
I do think it's a pretty fair complaint that it really feels like the software is fighting against you when you first encounter something absent from the musicbrainz database (especially if it's something fundamentally unsuited to be added to the database), but I'm not sure if there's an easy solution other than telling people "just hit the `import with existing metadata' button, it's totally fine" when they complain about it.
There is no canonical metadata for a fan recording of a concert or DIY CD-R, so you lose out on nothing by importing the files as they are today.
Once you're over the hump of the first import, beets is a fabulous tool for ingesting new music. It's well worth it.