I think we all would like to see rclone supporting Proton drive. But so far it was like pulling teeth. Somebody tried to reverse engineer Proton API and even if initially it was promising, longer term it turned to be unsustainable. Firstly it was one man show and with main actor moving to greener pastures there is nobody able or interested to carry on this path. Secondly with all this knowledge gone what we have is remote experiencing software rot. Simply as time passes it is less and less usable.
Obvious way forward would be at last Proton crew to decide that they are interested opening their system to 3rd party tools like rclone. Ideally supporting some industry standard API - S3, WebDAV or similar. Less ideal but still workable would be some custom API (official and documented). Latter it a bit like reinventing the wheel but if it makes them feel better, let it be.
But so far we have neither.
Feedback from other users, e.g.:
This is probably only thing Proton users can do short of voting with their feet if they do not get service they expect for their money.