If you’re using Gmail, and you’re considering alternatives for privacy reasons, then 100% without a doubt, objectively and unequivocably, Proton is the better choice of the two.
There are other email providers with privacy assurances, and yes, you can self-host, but don’t let perfection be the enemy of the good.
To address the trustworthiness of Proton directly: I’ve been a Proton user for about 10 years. It gets the job done. I have complaints, but privacy is not among them.
RE autoscaling: effective distributed systems design isn’t really language-dependent. Java apps can scale just as well as ones written in Go. That said, I can see there being a case for Java apps not making it as easy to build that way. There’s definitely a lot of mainframe/monolith-oriented patterns in both the standard library and in enterprise Java culture.
As for the job market and career investment, I’d say this:
I’ve written a lot of Java in my career and studied it in college, and I’ve written one app professionally and several hobby projects and utilities in Go. There’s a lot to like about it, regardless of its marketability on a resume.