odrive works pretty well if you’re looking for sync through Mac Finder. I think they also support Windows and Linux.
You can add both Digital Ocean and also Dropbox (and a bunch of other stuff they show on their website) accounts to their sync app.
I wanted to try something like you’re asking about–replicating from one storage to another. I think it’s possible since I saw something in their docs about being able to write a script to do it which can be scheduled to run on a periodic basis. But they only have a Windows powershell example right now and I’m on Mac so I haven’t had time to figure out how to do it.
If you’re looking for something a little more out-of-the box, I’m not sure what the best way is, but I do know that their app can at least make life easier (think of it like one sync app which can sync a bunch of different storages). It’s been very helpful from that regard for me, enough so that I uninstalled my other sync apps and just use that one.
P.S. I think the makers of Cyberduck have a sync product out called Mountain Duck, too. Cyberduck itself isn’t sync but it’s useful. I haven’t tried Mountain Duck yet… I would up finding odrive and sticking with it since their free version works really well for what I need.