I am all in on the Apple ecosystem with 5 Apple devices. My Apple ID which I’ve had since 2003 (iTunes) is attached to a Yahoo email address.

iCloud email just uses standard protocols and can be used with any email app. You just have to go on the web and create an “app specific password”

https://support.apple.com/en-us/102654

But why move to iCloud? I even use Apple’s Hide My Email with my yahoo account.