> I feel like I’ve tried every email client out there, but is there some solution I haven’t thought of of where I get a friendly GUI but can configure mail filtering in code (i.e something I can keep in git) rather than clicking around?

A couple jobs ago, before I gave up on processing all my mail, I had a perl script that grabbed headers from IMAP (with caching, because Exchange is slooooow), and filtered with code (basically a list of pairs of code refs that took a message => what folder to move the message if the code returned something truthy (or maybe the code returned a folder... not sure anymore). This let me do things that are hard in server side filters like moving things only when it's read.