The sloped roof is vented to the attic and should be vented at the other side which you may find when you dig a little deeper, it may be vented from one side or other, the strapping would help with that.
Anything new like this would be framed with 2x10s and strapping to give you air movement above 8" of fibreglass. You can add to the depth of the rafter.
If this roof slopes down towards the garage, you may find a vent wall built up there to draw air from the soffet above. If not something could be added to find air somewhere.
I am sure the roofers will have suggestion on what could be installed over foam above the flat roof.