Install the current spamassassin and turn on greylisting should get rid of 99%.

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.