I have not tried to do this, but I think you can set MaxMessages in your config to achieve something like this. If I'm understanding correctly, it essentially does a tail on your mailbox. The latest X messages will be kept, where X is the number after MaxMessages: https://manpages.debian.org/bullseye/isync/mbsync.1.en.html#...
I don't think I want all my email on my local machine, but I do think I want it all in the backups.
I couldn't see anything digging in.