This is completely missing the point of GP. You can't (unless something has changed) sync metadata without using their servers. webdav is for the data, not the metadata. If you don't personally care about privacy, fine, but that's irrelevant here.

Replicating the local Zotero folder across devices won't properly sync the metadata in the face of concurrent changes.

If I didn't care about privacy and freedom I would have just used a proprietary product to begin with. It's an incredibly disappointing situation to have dragged on like this for so many years.

Edit: Looks like nothing has changed. https://news.ycombinator.com/item?id=29774935