Strawberry or Clementine. I mean 100K entries in a database is nothing. Even for SQLite. You can add multiple library locations, this is no problem.

You probably want Strawberry as it is newer and maintained, but I still like Clementine for the extra features that Strawberry doesn't have yet. For you probably, not a big deal - things like podcast support, cloud support etc.

Strawberry could be a good contender.

I'd imagine mpd with one of many frontends would work well enough. You'd just need to use a dummy music library directory with symlinks to your four music storages for mpd to pick up and catalog everything.

I'm not sure what a "music jukebox" is, and how it's different from a music player, but I would recommend to try mpd. It should work with your collection, although I don't have personal experience with collections of this size. Some clients might also not have been designed to work with such collections, so probably you'd have to try several.

It doesn't have anything to do with the distro. With that many files, you're torturing the hell out of your disks, and your machine's memory. Depending on how the code is written, it depends on if this is a filesystem scan of the folders that are then imported to a local db which is looked up to go back and find the found files, or a simple approach which is to just scan the directory every time you go to open something.

I'd really think about properly organizing your files. If that's not an option, you can dig into the settings or code and find the hard limits set (probably for a good reason) on the number of files being scanned or imported.

Already mentioned, but Strawberry is worth checking out. If Quod Libet works, stick with that. I liked it when I used it.

mpd has a number of frontends.

No longer developed I think, but I'll mention gmusicbrowser as it used to be my go-to.

cmus is what I use now.

I also have a very large library and use Debian 12.5 so I'll bookmark this thread for later.