It sounds like Seafile would be worthwhile for you to look into, they do syncing quite well (based on a simplified version of git - its quite fast as they don't

It sounds like Seafile would be worthwhile for you to look into, they do syncing quite well (based on a simplified version of git - its quite fast as they don't have to up-/download the whole file if it changes), and sharing also works fine. Doesn't have a lot of other features, but what it does it does relatively well. There are a few issues as with everything, esp. setup and updating is a bit annoying, but overall I'd recommend it. Can also do end to end encryption (with a few caveats, but IMHO good enough).