At least for insync 3.0.x in windows, outside of the base directories, there is an insync context menu item to sync files/folders. This includes items in mapped network drives and usb devices. I have no idea how it functions or if it does it well as I have not tested it. But this could be an alternative, ie syncing the symlinked folders directly rather than via the symlink.
Similar thread with possible solution
I’m definitely with you on the google photos issue. And to only provide one way sync in the native google app and not provide an API on top of it is asinine.