Definitely more advanced than what I do! I've also been collecting for a long time, and I've just been storing the media. I went from Windows/x86-64 to Mac/ARM64 a few years back, and I have kinda limited options that way. I would have just gone to Linux if my computers didn't die, but at the time, Apple made sense.

I do Apple Music, but, that doesn't answer the question. I also self-host my music via a Plex server. I got Plex Pass for well under $100 on sale years and years ago. Much harder to recommend now. Plexamp is awesome for music, and I also use Prologue (iOS app) for audiobooks hosted on Plex.

For tagging, I use Mp3Tag and tag manually. It was free on Windows, but the guy wants $30 for it on Mac. It's a long story as to why, but after he was kind enough to explain it to me... I paid the man. (For my part, I'd used it for free for many years and I got a lot of miles out of it, and still do, and it's a good, well maintained app from a solo developer.) There are free alternatives. Not so many good ones on Mac (part of why I paid). For album art, I get them from Apple using this handy site (not mine): https://bendodson.com/projects/itunes-artwork-finder/ — you can use 600px or you can use either 2000px or 3000px, I forget which. I'm good with 600px. I think Apple uses the smaller size for phones and the bigger one for TVs and Macs (especially as iMacs have 5K screens now).

My library might also not be as big as yours is. Plex says I have 14,462 tracks indexed. Maybe those are rookie numbers, I dunno. I pretty much have everything I want. All I'd really want now is to take my Apple Music playlists and download them to m4a 192k or similar. I have a few decade playlists that are like 500-600 songs each (80s, 90s, and 00s).