Just FYI possibly for future sync capabilities. The people that make the privacy based Proton Email and Proton VPN have Proton Drive in beta. I contacted them to see if they supported webDAV and they said "... currently, we do not have plans on integrating WebDav. The reason why is because we use a proprietary protocol to maintain end to end encryption, and WebDav does not fit into our current model."
I don't know if they have other ways for syncing data but syncing via Proton may come up as a request in the future.
Due to no WebDAV, I didn't purchase a Proton account at this time.