I don't doubt your experience, but I've had the same pattern (gmail.com email spammed to hell, custom domain -> no spam) except that I'm in a grandfathered Goog

I don't doubt your experience, but I've had the same pattern (gmail.com email spammed to hell, custom domain -> no spam) except that I'm in a grandfathered Google Workspaces account. I also do the custom email trick, and I'm very liberal about handing out those (makes remembering accounts easier as well), but no dice. I'm convinced that companies filter out custom domain email addresses when they share and spam user data.