Leaving Gmail for Mailbox.org

I don't understand the logic. I mean if you hate Google, or you specifically want to avoid Google having your data sure, but whichever email provider you choose will have at some point access to your plaintext email and they may choose to store it. There's no such thing as real end-to-end encrypted email unless the sender actively does so or you run the server yourself (and be online to do the TLS handshake when someone connects to your domain's MX address).

Another concern about anything social is that there are at least two sides in a conversation and whoever leaks the data to a third party will compromise privacy of all so it is really hard to prevent your email from getting to Gmail servers one way or another.

If my wife and I both use Proton, or I correspond with a bank or company that doesn’t use a Google product, how would that possibly get into Google’s hands? You way oversimplified the possibilities here.