I am using Google Workspace.
Starter account, for 5,5 USD per month for one users this ist enough.
I have setup the domain in google and on mailcow. As mx record I am using my mailcow. Incoming mails are delivered directly to mailcow. Outgoing mails are delivered from mailcow via google smtp relay. mails addressed to configured domains in mailcow never leave mailcow and are delivered internally without smtp relay.
With this setup I can create mailboxes how much I want without more costs on google side.
The external delivery I perfect. Gmail servers have from my point of view a good reputation.