Looking fantastic well done, been really enjoying seeing the updates.
When I boarded out my detached garage extension (Basically another 4m extension on the end) I used battened the brick walls with 2x1 and screwed 75mm PIR insulation to the battens, then used foil tape to create the continuous vapour barrier before covering with OSB. I had a double layer of PIR on the ceiling, put 100mm between the ceiling joists, filled in any gaps with expanding foam and then overlaid the whole lot with 25mm before covering with 12mm OSB.