I had one of these moments where a seemingly obvious insight dawned on me: Using procmail rules I can set up as many maildirs as I want and automatically move mails there. Not just spam, but also every mailing list into its own dedicated directory. Less fiddly than integrating notmuch/mu into mutt, too.
Editing sieve rules in the web UI is kind of bothersome. I've considered implementing an Emacs package for that, then discovered M-x sieve-manage is a thing. That leaves doing auth in a more automated way.
A lonely little town in the wider world of the fediverse.