From this[0] report from proton mail, 74% of European email is routed through USA servers. So even if you care, the person you are sending to or receiving from probably does not. So the US companies are still seeing it.
For the use case of you and your wife, you don't really need to self host a mail server, you just need a file server, unless you're forwarding each other emails you got externally, in which case, someone probably already read it.
My overall point is that email requires at least two parties to have access to the content, oftentimes more than two, and chances are one of those parties is not privacy conscious, which breaks any privacy you think you might have.
For the use case of you and your wife, you don't really need to self host a mail server, you just need a file server, unless you're forwarding each other emails you got externally, in which case, someone probably already read it.
My overall point is that email requires at least two parties to have access to the content, oftentimes more than two, and chances are one of those parties is not privacy conscious, which breaks any privacy you think you might have.
[0] https://proton.me/business/europe-tech-watch