Fastmail will support your domain name and has a very good importer from Gmail. You don’t have to transition everything all at once. One thing that helps is to

Fastmail will support your domain name and has a very good importer from Gmail. You don’t have to transition everything all at once. One thing that helps is to use different apps to access Gmail (like the official app) and Fastmail (your phone’s built-in Mail app), and if anything arrives @gmail.com, either unsubscribe or update the address.