I want to do this too, and have a feeling that it's not as hard or time-consuming as it seems. 15 years ago, all my music lived in a /Music folder and I could play anything in there, instantly. It should be easy to just move that folder to a networked drive, get some sort of mp3 player app on my phone/devices, and point it at that folder. If the app is allowed to download files as well, that's even better. Otherwise, plugging in my phone/mp3 player and uploading songs manually was never particularly difficult, even back then.
If I remember correctly, all my playlists were really just text files used by Windows Media Player or iTunes, so it should be easy to support that type of functionality as well.
You can more or less do this with apps that will stream your library off Google Drive. The one I tried demanded permissions to read everything in my Google Drive which seemed too dangerous, but if you had a separate cloud drive somewhere you could set it up pretty easily.
I believe it's too risky to have DMCA-able content in Google Drive.
Maybe, though I will point out that Google did/does offer this as a service:
https://support.google.com/youtubemusic/answer/9716522?hl=en
The VLC app can read and play from networked drives, at least on my iPad.
Run a DLNA server and you client options grow.