I'm no expert, I followed the advice of Liam at https://www.youtube.com/@thegardenroomguru who has built many, many garden rooms. Plus plenty of others on YouTube all seem to be using 50mm PIR in the walls.That looks great and you've got to love a spreadsheet....
I'd appreciate your thoughts on using 50mm PIR over anything deeper in the walls given you've used 4x2's for the uprights and the electrical cabling seems to be run through surface-mounted trunking so doesn't appear to be within the airgap behind the OSB, suggesting that there's space.
I ask this as I'm currently helping our eldest with a smaller (than yours) garden room where he can occasionally work and set up his road bike training frame but where we've considered 75mm PIR too.
I believe the company he's looking to use for the PIR are called Pinks where the difference in cost between each sheet is nearly £9 (£20 vs.£28.74) and I frankly don't feel I've enough experience to persuade him that the extra costs will be more beneficial in the long run. Perhaps it's just illogical on my part but I want to persuade him to get the 75mm and just fund it for him but if there's no real difference and he's happy with 50mm, I don't want to interfere.
All the T&E runs in front of the PIR, which is pushed hard against the outer OSB.
So yes, you get an air gap between the PIR and the inner OSB. See attached photo.
I was told not to waste my time with a plastic sheet vapor barrier, as is often used when plasterboarding the walls instead of OSB in this type of construction. It is a low-occupancy building, not a kitchen, and the inner OSB is glued together with waterproofing adhesive, so it is a sufficient vapor barrier. Time will tell
Also attached is an image of the Excel I used to make sure I was under the 2.5M height for permitted development.
As this was my first construction project, it came in handy!