If tweaking spam filters and deleting spam is your idea of fun, then run your own email server.
Thanks for bringing up the painful memories of adding new filters each night. So glad those days are over for me.
edit: Although some training (i.e. manually moving spam to IMAP-Junk) was necessary during the first few months.
The only spam mitigation I use is greylisting and blacklisting.
Failover is manual. I would create a new vm, install debian stable and run a script I've prepared a script that installs packages, copies config files and creates users. Downtime with email is not a big deal, servers will retrying sending for a few days. If you have users complaining to you it's obviously a bigger deal.
Then I'd block the spam only domains like .click, .link, .party, .top, .webcam, .xyz, and .stream.
That should be enough to get you close to zero, sure having spamassin learn based on your Junk folder will help improve things.
I've run a mail server for 20+ years and never had a spam issue, periodically I do updates spam assassin of course.
since the machine is a webhost as well, also hooked up spamc to mysql for better SA performance while still keeping per-user procmail + sa config