Imapsync maybe? I haven't personally used it, though I did previously buy it thinking it would download to local machine -- it transfers between accounts using IMAP which wasn't my intent but sounds like it is yours.
I realized that I was just reading about GMail backups a few days. Here are some repos that I happened to star on GH.
* https://github.com/gauteh/lieer
* https://github.com/imapsync/imapsync
* https://github.com/joeyates/imap-backupTL; DR: Seems to work BUT may have issues with SPF/DKIM etc.
Looking for anyone else on HN to chime in with their experience using iCloud custom domains.