Wait, this is terrible news. I was planning to migrate off of Gmail using that feature

It does work great generally. Here's what changing your foward-to looks like: https://imgur.com/a/feSBCCN

I migrated family email hosting from Google Workspace to Apple iCloud+ Custom Email Domains a few years ago and it's been smooth sailing. Few people realize that Apple has been a high-volume email host since long before Google Mail. This is the guide I used: https://blah.cloud/miscellaneous/migrating-google-workspaces...