That's what I mean by continuous automation, using rules (constraints) to dictate how the library is managed (in your case, what beets does, e.g. auto download art of the given size and format, constrain the genre etc etc).
Nice approach. I'm the developer of https://www.blisshq.com/ and other projects. The key insight there is that managing large collections is easier with rules applied continuously, rather than thoughts-in-the-moment "I'll run a batch command with a tagger".
You could have the rules encoded in your config (or the DB) and apply those. For example, someone may want a "portable mirror" of their gold library, but that means transcoding AND chopping the embedded album art to size (or maybe their player has max resolutions).
I guess having the DB means you could potentially version control the metadata changes?
At the back end of the 90s I did some work with self-organizing maps - collapsing multi dimensional space into a 2D map. I think it's interesting as a data visualisation approach. This looks similar but I'm totally out of date with where SOM went.
When you say 128d space, what are the data that are represented, exactly?
Not only do A&A offer good speeds, their customer service is exceptional. There are chat rooms on which engineers and enthusiasts reside and if you phone them you get an engineer.
OP could use something like mp3fs to make the transition from MP3 to FLAC less painful where MP3s are still required - it's a lazy mirror of a FLAC collection to MP3.
(Disclaimer: this relates to the commercial project I run, but it is directly answering the parent)
It's not self hosted, more a middle-ground between rented Spotify and self hosted data sovereignty, but this is what we do at https://asti.ga . You store your music in some Internet-accessible storage, such as any S3 compatible endpoint, and Astiga connects and streams your library (and provides offline etc etc). AMA.
You could get images from multiple perspectives from the Cover Art Archive (via MusicBrainz). E.g. https://musicbrainz.org/release/a4f190d5-c752-4ebd-8d2f-925a... . It would be very cool if the spine could be rendered, although it's not always split from the back image (I guess because of the post processing after scanning).
The tragedy of of this is that these are improvements that would actually improve life in these houses - making them healthier, more comfortable. Trouble is, retrofit is expensive.
Another source are good ol' CDs. If you know where to look you can get them cheap too. Specialist charity shops that specialise in records and CDs in student or wealthy areas are good, as are second hand stores online.
You can get Android based music players (DAPs e.g. https://www.reddit.com/r/DigitalAudioPlayer/ ) which you could install software which streams when online, uses local storage when offline. Look into the Subsonic protocol - many music servers support this and it has built in support for this online/offline dichotomy.