

In general, setting up a client with POP3 rather than IMAP will move all the emails from the inbox to the client. Once they are in the client you can decide how to backup the archive, there’s plenty of options with varying performance and memory footprint. That sounds pretty much like what you would like to do in general.
Just for completeness: That of course also means that syncing to several devices and backups are now your problems to solve.
In any case, the initial download will take a long time either way if you have 20+ years and 10s of GBs of data. No way around that.

Moved to Apple Music after Rogan started airing on Spotify. Hence Apple Music on the phone, streaming through Cider on Arch otherwise
Edit: I could not work with local files or my own file server because of the breadth and amount of music I listen to. I’d have to have a massive collection and constantly keep buying 🤑