Hint: You can sync the library for free by making an account. You need the "Zotero Storage" only if you want to sync the files as well. However, you can simply sync the folder ~/Zotero/storage using SyncThing. You could also use WebDAV, but SyncThing is just so much easier (set up & forget). The IDs/Metadata is consistent among syncs. :)
Of course, that doesn't resolve the privacy concerns, but personally idc about that.
edit: It seems like you might be able to sync everything by just putting ~/Zotero/ into something like SyncThing, avoiding the Zotero servers. But you'd probably have to be careful only opening the app after syncing has been completed. I haven't tested this though.
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.
"Just fork it" is dismissive and misses the point. The criticism remains valid even if the maintainers refuse to act on it for whatever reason. Even if I were to invest the time to fork the project, the criticism would _still_ remain valid as long as upstream didn't take steps to fix the problem.
I‘m on windows, and here it is still possible. Try checking your settings again, it is a bit hidden: goto settings/sync, then file-sync -> via WebDAV is available in a Dropdown menu.
Of course, that doesn't resolve the privacy concerns, but personally idc about that.
edit: It seems like you might be able to sync everything by just putting ~/Zotero/ into something like SyncThing, avoiding the Zotero servers. But you'd probably have to be careful only opening the app after syncing has been completed. I haven't tested this though.