For those who really want to dive into this, ChronoSync have released version 11 in public beta! You can download it and find out more Here
However, it looks like ChronoSync is using Apple's "file provider" APIs to do this, which is very disappointing and not ideal, because it inherently creates the same problems outlined in the first few comments in this thread.
If using the new File Provider extensions, Apple forces any content that is located in cloud storage, like Dropbox etc, to be downloaded in the background on your hard drive, just like the native Dropbox app does. I was hoping that ChronoSync would get around that by allowing you to DIRECTLY link your Dropbox account to ChronoSync as a "connection target" just like you can with Amazon S3 and Google Cloud Storage/Backblaze currently. But instead, they're relying on Apple's framework to make the connection between cloud services, presumably because they don't want to have to develop an applet for every cloud provider. ChronoSync does allow you to remove downloaded/uploaded files in the background after it's done syncing using this new file provider framework, which is a nice workaround for the above, but it would still be much more efficient to directly connect the destination service. Dropbox also has not rolled out the file provider capability to all users yet, so for me, I can't test it with ChronoSync because I can't enable the feature in my Dropbox account yet. All very interesting; I'm going to email ChronoSync and ask them if there's a specific reason why they're going this file provider route, and if there's any chance they will implement direct connections to cloud service providers in the future. I'll report here if I get anywhere. 