Rsync only works for the files at the moment but it cannot touch the photos which is a shame. I feel bad opening up a tickets in each of my two accounts asking to wipe both drives.

Currently de-Googling and un-Microsofting. In the first account I dumped Google Takeout Extracts while syncing from two phones with a sprinkle of OneDrive and have duplicates and screenshots galore. Thinking I've learned from that in my second account (I love Proton Pass), I deduped and filtered out screen shots locally but over 500 albums were created for each year and underneath that each day (!) and when I started sync those folders migrated over in essence ending the second operation before anything ever started.

Would have to batch delete from what I've seen around 2500 photos at a time out of about maybe 13,000 using the web client in the first account and then from the second manually delete each of the 500 albums since more will be automatically created (I do like that feature).

rclone works well with ProtonDrive (since v1.74.2).

I wouldn't deliver solution that is has to be rsync or whatsoever. But I agree that a solution for Linux users is definitely crucial.

Indeed having a drive (proton) which connects and sync (with e2e) in linux would be essentiaal for using on all of my machines.

Maintaining the E2E encryption is more important to me than allowing rsync or rclone.
To me, a Drive bridge is a better idea than a direct API to my data. Maybe using a VPN-like structure from a local service to Proton. Then rsync or rclone could connect to localhost at an arbitrary port.
Of course, if you have an official bridge, that bridge could also have a sync option/setting so that the user would not need to drop to CLI and use a third app.

Really important as I am looking at replicating data for resilience!

So glad I found this community. I thought I was the only one having issues with rclone and Proton Drive. Want to migrate off Google Drive but can't until this is resolved.

1000% agree on this. Proton doesn't need to reinvent the wheel. Work with existing open source tools where possible. If anything, this is not just less work for Proton, but they could end up contributing to open source projects, making them better for everyone, but on top of all of this, it's even better from a privacy and security perspective.

Just in case anyone else comes here, rclone tries to solve this problem, but I have found it to be very unreliable, often failing, creating duplicates, etc. I suspect this is because Proton's API is poorly implemented, not because of rclone itself.

That would be fantastic! But I would be happy if there was at least a Linux client like under Windows. Maybe Proton can give you some news about the status here. Apparently they are working on it.