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.