I did the same thing. Custom domain with Protonmail. Ended up frustrated that I couldn't set up forwarding rules and alike. I realised I didn't need the privacy

I did the same thing. Custom domain with Protonmail. Ended up frustrated that I couldn't set up forwarding rules and alike. I realised I didn't need the privacy as much as I needed features. Swapping from Protonmail to Fastmail was super easy. Exported all my email, imported into Fastmail, and swapped over the DNS records. Took me a couple of hours in between other tasks.