I suggested using a virtual drive
a cloud service, not instead of. Your comment implies that it’s one or the other.
There’s still an issue if an identical virtual drive cannot be created on both platforms.
I guess I’m missing the reason as to why you need the virtual drive if you are syncing external files via Dropbox? For example on Windows my path to my Dropbox is C:\Users\USERNAME\Dropbox\ my username is the same on both my Windows Desktop and Laptop, so when I use external attachments they are the same path on all my Windows devices. As far as I am aware there isn’t an issue with syncing external files via cloud sync only TheBrain databases. If the path isn’t different and there isn’t an issue with external files syncing via cloud service what would be the advantage of using a virtual drive? If for example macOS mounted drives like Windows Z:\ then I could see a definite advantage because paths would be the same.
macOS 10.14.6 Windows 10 1803