Email is not something private by design. At least Google is a competent company when it comes to provide such services.

They don't read your email, they scan it. These things are semantically and practically different. Reading, to me, means human comprehension. It's not like some Google employee is going through my messages. Scanning is just robots putting keywords in a database to target ads. Which is annoying, but it's not really the same, privacy-wise.

You don't have to be a GMail user for Google to end up with a copy of a large percentage of your eMail.

Because lots of users do have GMail a lot of your messages will end up on their servers.

However, that's not to say it's not a good idea to practice good hygiene yourself and find a provider that suits your goals.

If not Google then who? Fastmail? Protonmail? Someone will always read our emails, I just don't see a way around it. We kinda live in a world based on trust, maybe someday that will change, but probably not any time soon.