Just finished moving my email from Proton to Fastmail. The documentation for this makes it sound like the only option for importing your email from Proton is by hand via .eml files in batches of 1,000. :yikes:
But I was able to use Thunderbird, connected via Proton Bridge, to create MBOX files for each folder and then use the "HEY" import feature in Fastmail. Much easier.
Thunderbird has an Export tool called "ImportExport Tools NG" that made this possible. Hooray!