Aw schucks. I moved to Proton half a year ago in an effort to de-Google (and de-US) myself. Now this. Time to write a message to customer service explaining my concerns.
I'm in the other boat, I've been pondering de-Googling to either FastMail or ProtonMail, and struggled to make a decision.
Now it's been made for me and I can plan my migration to FastMail :-).
You’re probably better off switching to one of the many good alternatives.
Fastmail is good, I've used it a few years.