When I was following Synology communities closer, the common wisdom was that the expansion units weren’t great in either performance, stability or cost, and you were better off buying a new, bigger unit and then selling your old one used to recoup the cost difference.

I’m also in the same position, I have a DS918+ that is full. It’s also 6 years old and probably on the tail end of getting software updates so I’m weighing my choices…