I've tried Hey, it's nice, but you're stuck with their workflow.
I decided to reproduce their workflow inside of Fastmail. Worked well and now I adapted it for my needs. Something I couldn't have done with Hey.
Even today, I'm exploring Proton and I'm finding that some basic features offered by Fastmail are not available in Proton. The idea of encrypted emails is nice, but I'm not sacrificing some features that I use.
There is another important feature too that I need, but I don't know if proton supports it. Fastmail currently manage the emails from 2 of my domains. I also supply an email address from one of those domains to each member of my family. I need to be able to forward every email received to a specific address to a Gmail address. The emails must skip my mailbox completely and not look as a simple forwarded email in their Gmail.