First set up the Synology Drive Server on the NAS and Synology Drive app on the Mac. Do not yet link Mac to Synology Drive server. Ensure you create a folder called Dropbox in /homes/andy.
Then setup Cloudsync and point your Dropbox to the Dropbox folder you just created and magic will happen. Your dropbox will start syncing with the NAS. To avoid any issues let it sync completely. The icon in top right DSM will tell you when its ready.
Now set up selective syncs in Synology Drive by linking to the Dropbox folder on your NAS and the stuff you need will replicate to your PC. After that changes will replicate seamlessly between Mac, NAS and Dropbox.
I only use select folders to actually sync. If I need something more I either mount the NAS folder or use Filestation in DSM. Alternatively, I log into Dropbox on the web.