Hello John and welcome to ER! Glad to see another project going.
Do you have a ridge vent there? Its hard to tell. I'd recommend adding in that 50mm vent space all the way to the peak. Then, I'd insulate the back side of that vent all the way and encase the entire room. You should be able to do this so you don't have any leaks. Leaks are the #1 way to loose heat. Once you seal everything up then you can start worrying about how to insulate it the best you can.