A few people commenting that some of their collection "doesn't exist in any DB", the best way to fix it is to add it to Musicbrainz[0] yourself!
I have found that adding things to Musicbrainz is actually pretty easy (and if you are so inclined like me, pretty rewarding and fun).
Streaming releases (and Bandcamp) you simply drop the release URL into Harmony[1] and it does most of the work for you.
Musicbrainz can represent nearly everything musically related and its all freely licensed, a very cool thing to exist.
Most (non-destructive) edits are auto-applied, whilst the rest go through a 7 day voting period (they are still applied by default unless someone votes against). The barrier to entry is very low.
You're still assuming that all of a person's music metadata _belongs_ in a global database. Some of my collection includes:
-"frankenstein" musical soundtracks where I've assembled my favorite version of each song from all the various published recordings. Sometimes I've even edited pieces of different recordings together into a hybrid track.
-a soundboard recording of my friend's high school talent show performance
-Music I've personally recorded from video games or other random sources
-Songs where I've edited out parts I don't like (such as overly-long drum solos) leaving just the parts I do.
I've organized these things in a way that makes sense for me, not for the world.
This is like a bookshelf that can only store books with an ISBN that can be classified in the Dewey decimal system. Too bad about your family photo scrapbooks or your personal sketchbooks!
What I think they're doing (and what I've done) is add music that wasn't already in the global database to it. For example, a promotional CD, or releases from a small publisher, etc.
In these cases, you're not adding personal metadata or mixtapes.
That said, definitely do self organization if it works for you. Most of your examples seem like they're suitable for custom playlists.
I have similar reservations about custom tags. My collection makes heavy use of non-standard tags, such as performer:<instrument>, opus, key, subtitle, or style. I can't find anything in the documentation about which tags are supported, or for which formats (I keep A/V registrations in Matroska containers, using the same tagging convention -- like using performer:<role> to save actor credits).
There is value for me in having a central database for this data, for example to find misspellings of the same name. But the fear of having 20 years of custom data entry destroyed by an overzealous tool makes me very hesitant to even try a solution like this.
Wow I've needed harmony for years, thanks for sharing! My dumb ass was filling out the Musicbrainz by hand for like two months before I just gave up on beets.
You'll want to bookmark Harmony even if you're using Picard since it's driven by the same database. Happily, once you add an item using Harmony, Picard can find your just-entered release almost immediately via the Release ID of the release you just created.
I find myself needing to create releases for ~10% of the albums I tag, and Harmony is a game-changer for that.
I have found that adding things to Musicbrainz is actually pretty easy (and if you are so inclined like me, pretty rewarding and fun).
Streaming releases (and Bandcamp) you simply drop the release URL into Harmony[1] and it does most of the work for you.
Musicbrainz can represent nearly everything musically related and its all freely licensed, a very cool thing to exist.
Most (non-destructive) edits are auto-applied, whilst the rest go through a 7 day voting period (they are still applied by default unless someone votes against). The barrier to entry is very low.
0. https://musicbrainz.org/
1. https://harmony.pulsewidth.org.uk/