If you want to sync larger Zotero libraries for free between computers you can use SyncThing to sync the libraries and then let Zotero sync up the database. SyncThing isn't cloud based, so your machines have to be on at the same time, but otherwise it works great.

I posted instructions on the Zotero forums a while back: https://forums.zotero.org/discussion/50191/syncing-zotero-wi...