I do the same as the poster above, fastmail supports it directly and makes it very easy to manage. All you have to do is bring your own domain (they'll even manage your DKIM/SPF records etc as necessary if you want).

Edit: Apparently you can also purchase a domain directly through them if you prefer, although you have to be a paying customer for 7 days first https://www.fastmail.com/how-to/email-for-your-domain/

I use simplelogin with proton for that, they give you a few subdomains to do the same.

I have the same setup via GSuite.