I have Proton "Unlimited" since last year. I never thought of using Drive as a computer back-up option. I'm still using the good old local HDD for that. I just use Drive like a safe for scanned documents. So there was never any upload problems because of the size. It just seemed like a good idea to use it for Joplin, but I know absolutely nothing about the workings of WebDAV. So I guess I was wrong.

I found protonmail.uservoice.com last night, and somebody had already requested Webdav support in Nov 2022. It has just over 500 votes. So it may be looked at at some time in the future.
You can find it at the following link if you are interested:
https://protonmail.uservoice.com/forums/932839-proton-drive/suggestions/45915592-webdav-support