I was on Gmail with a custom domain for a good 7 years or so and in this last year I switched to iCloud mail and have been happy with it. I trust apple more than I do google when it comes to email privacy. And I think what really pulled me over to having apple host my mail is the private relay feature which is super cool.
I use iCloud mail because everything I own is Apple. You just can’t beat how it syncs between my phone, Watch, iPad & MacBook. FWIW, the native mail app is the only one that can compose a new email from scratch on an Apple Watch. I’m all in with Apple services.