I successfully migrated to ProtonMail using a strangulation pattern.
I set up a forwarding policy from GMail -> Protonmail. Whenever I would receive a forwarded email, Id go to the sender and update my email address. After about 6 months the only email coming from my Gmail account was spam.
Then I turned off my forwarding policy and noticed something: I don't really get spam anymore. I don't know what it is about Gmail but it receives several of orders of magnitude more spam than any of my other email accounts. To drive that point home, to crack down on spam, I setup my own domain on protonmail and configured a catch-all. Now everyone gets their own email address (like homedepot@mydomain.com). It lets me reverse track who is sharing my personal info with who for marketing purposes. Turns out: in the 5 years I've been using Protonmail I've had two cases of someone sharing my email address. I had assumed all my spam was from people sharing my email - turns out it was just a Gmail problem.
If your experience ends up being the same as mine, the time you save not dealing with spam on Gmail will cover your migration costs.
I can confirm what this poster said about their business emails not actually getting shared around, but that spam just all goes to gmail... I did a very similar setup and got similar results, but I'm using FastMail, though I'm sure protonmail and mailbox.org would work for this, had I chosen them as well.
Sending decrypted information over an encrypted line makes it relatively much easier to reverse-engineer the private key. If Google has, say, the contents of an email via GMail, and surveillance over the transmission line that carries the encrypted version of that, they would have not much trouble cracking your Protonmail key. It's unlikely that they would gain access to Protonmail's secure servers, but if they can surveil traffic going into and out of Protonmail's servers, they can decrypt the messages they know the keys for. They own more than a few installations and high-throughput (e.g. undersea) cables and it doesn't seem far-fetched to assume they have built systems for extracting information from the massive bitstreams, especially considering all we know about NSA surveillance programs.
I set up a forwarding policy from GMail -> Protonmail. Whenever I would receive a forwarded email, Id go to the sender and update my email address. After about 6 months the only email coming from my Gmail account was spam.
Then I turned off my forwarding policy and noticed something: I don't really get spam anymore. I don't know what it is about Gmail but it receives several of orders of magnitude more spam than any of my other email accounts. To drive that point home, to crack down on spam, I setup my own domain on protonmail and configured a catch-all. Now everyone gets their own email address (like homedepot@mydomain.com). It lets me reverse track who is sharing my personal info with who for marketing purposes. Turns out: in the 5 years I've been using Protonmail I've had two cases of someone sharing my email address. I had assumed all my spam was from people sharing my email - turns out it was just a Gmail problem.
If your experience ends up being the same as mine, the time you save not dealing with spam on Gmail will cover your migration costs.