So predictably this was not as straightforward as hoped.
Mac OS X.11 Sierra just released, and the main change is much more streamlined iCloud integration which is what I wanted. However, if one copies the Dropbox folder into the iCloud documents folder, it just sits there and refuses to sync (the cloud icon gets a line through it to signify that it can’t be synced). I think this is something to do with the fact that the Dropbox folder isn’t a ‘real’ folder as far as OSX is concerned – It just tricks you into thinking it is.
However I think I have gotten around my issue with the use of Symlinks in the dropbox folder.
For anyone who stumbles on this and wants to know how it’s done (unlikely I guess):
Enable iCloud Documents sharing (in the iCloud pane of System Prefs)
Open Terminal and navigate to your dropbox folder
cd /Users/<yourname>/Dropbox
Then create a symlink here to link to the /Documents folder (which is now sitting happily in the iCloud)
ln -s /Users/<yourname>/Documents
Then drag everything from the Dropbox folder (apart from the newly-created symlink) into the iCloud Documents folder
The only problem with this is that Dropbox thinks everything is new (so it’s deleting everything in the cloud and re-uploading everything again) whilst iCloud is also simultaneously trying to upload the new 4Gb folder. So I’m basically uploading 8Gb until both services are synced and happy 🙄
Anyway, should be sorted by the morning 🙂