This isn’t the perfect solution but hydroxide is a “A third-party, open-source ProtonMail CardDAV, IMAP and SMTP bridge” that works great on the Reform. I am able add my Protonmail account to Thunderbird and Evolution and it works fine.
There are some issues with certain encrypted emails, but for the most part works well.
Just make sure you read the github page and are good with the limitations and current status.