Syncing with Proton Drive works great on the desktop but unfortunately, on mobile not so much since there is no file system option where you can point to a directly which could be your Proton Drive since Files (on iOS) supports & recognizes your Proton Drive as a folder option. So sadly a non-starter since syncing between desktop AND mobile is a key point of syncing in the first place 
BTW - I understand this is most likely a limitation of iOS and not Joplin.
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