For me, yeah, it sucks knowing if you send an email to a gmail user then your message is in google still. But, I take comfort in knowing I’m not signing into or using google myself. Just be aware your emails aren’t protected at all. I agree with the other commenter - use whatever is convenient. Emails are never a very private communication form, either way. I also use proton but have been a bit conflicted since they continue pushing AI integration and I’m not pleased knowing my money is helping fund that for them.

A Fairphone with GrapheneOS would be the tops