As far as I know, tested, and using right now - rclone (through Round Sync) on Android support Proton drive. And it uses the same core as normal Linux rclone.
So yes, there is a client - rclone. And believe me, my own Nextcloud and
pronton drives are accessed through rclone. Most clients suck
For the moment. Round Sync on Android (use rclone) and rclone on Linux to just mount it. Better than any stupid client tbh