I get that. If (and when) I change email providers it'll be a pain. My gripe with a custom domain is that it'd uniquely identify me. I prefer to hide behind fastmail.
Ah I didn't realize cashback was US and Canada only. Purchasing over $6000/year will cover the fee, which isn't cheap if you don't do much shopping there. The rewards site has a table with reward estimates: https://www.costco.com/executive-rewards.html