We are in Marine Zone 4

Our walls ( see attached) are framed with 2×6-16″OC with a continuous 2 x 12 header.  The wall are finished with 1″ of XPS on the outside but on the inside there isn’t much area to fill the walls with insulation. 

What are the options to insulate/reduce thermal bridging on the internal side the wall that add less than 3/4″ to the depth of the wall?

Thank You
John