ᐅ Insulating a slanted wall / cavity in front of or behind insulation

Hello,

I am currently renovating our garden shed (wooden structure), which we recently acquired and had to completely gut first.
Now I have a question about insulation, as I have the following fundamental problem:

Unfortunately, the exterior walls are completely uneven—both in the floor plan lines and vertically. For the interior cladding, I need to build a furring strip framework that is straight. However, this will result in gaps between the interior wall cladding and the exterior wall varying from 8 cm to 22 cm (3 inches to 9 inches).

My question: Is it better to install the insulation directly against the exterior wall or directly against the interior wall cladding? In other words, where is it smarter to leave the cavity due to the varying distance? Most walls and ceilings only allow for insulation up to 8 cm (3 inches) thick. This would mean that, with the uneven walls, there could be up to 14 cm (5 ½ inches) of air between the insulation and the interior or exterior wall...
If necessary, here is some additional information:
The exterior walls are constructed from inside to outside as follows: wooden battens, expanded polystyrene (EPS), roofing felt, tongue-and-groove cladding. Inside, I plan to install a vapor retarder over the insulation and then cover the wall with 12 mm (½ inch) plywood panels.

Thank you for your suggestions and recommendations.