I kinda struggle to believe it's that difficult. I mean, Tresorit has a pretty good and functional Linux client. What have they done which makes it sustainable for them?
Filen.io also has a pure sync-client, which is distributed as an AppImage. This also works, but the FUSE integration Tresorit provides is quite awesome and performing quite decently.
I would actually recommend Proton to start the development on an older Linux distro. Like RHEL/Alma/Rocky 9 or Debian 11 (which is EOL, though) and make it run there. Moving from that distro to newer distros will then go smother and you'll get other distros supported quicker.
The mistake too many Linux efforts does is to take the "latest and greatest" distro version - often coupled with what a single Linux developer considers the "most used distro" and then hits lots of challenging when needing to support older distros. That's going to be painful.
@protonprivacy Please take note and forward to Andy and other managers.
@BastingChemina Proton Drive on Synology? I suspect that to be built on top of rclone. Unfortunately, the current rclone approach is incredibly slow.