Instance segmentation performance depending on the number of classes
The process you have used it spot on. Your problem: Give more importance to one class than the rest will have to be hard-coded in. There will be no pre-trained networks for that class especially.
One thing to note could be that assuming you have to segment roads in a road scene problem. You can create a function which can bias the weights towards the road specifically. This is just a thought, I haven't implemented it myself!