The only thing I know is RClone or something which I am not technically advanced enough to use. If I am paying for a Proton subscription, I shouldn't have to make any work arounds. They should ship the full suite by default.
They don't take money from investors but grow organically, which limits their resources quite a bit. With more users being on other platforms and Linux being a bit more complex when it comes to amount of possible filesystem and other combinations I see why it takes them a while. Iirc they also do e2e encryption of (meta-)data which does increase complexity.
Hopefully they'll finish it at some point, as it's been a long time since they announced Proton Drive. As I'm not paying for Proton, I understand a paying long-time subscriber might not share my acceptance of them zaking their time.
While there is no denying that, their decisions of not releasing flatpaks and instead releasing .deb and .rpm files is something contradictory as flatpak is a literal solution to make unified packaging formats on all Linux distros, but Proton is instead focusing on package manager versions which just makes their own life more difficult. They have done this with the Protonmail beta release on Linux btw.