Don’t wish to sound critical
I'll get my plans up on here when they are a bit more complete
You’re 2 years in, and only now drafting up a plan! I'm really surprised you haven’t detailed all the elements already.
peoples builds are spacing their studs at 610 centres for the internal 11mm osb cladding, is this not a bit far leaving the osb unsupported over 610mm? Limits fixing points for shelving too.
The simple solution would be using a thicker sheet, if you intend hanging shelves etc.
Just for some clarity: principally the OSB makes up the structural element to help resist racking, TF makers use 9mm which is smaller @ 1197 x 2397mm and is designed to have a margin gap around it to allow for expansion, but, it is fitted to the outside (separate debate there!!)
Using this then sets the stud gauge to 600mm, and coordinates with plasterboard, which is 1200 x 2400mm.
In your case closing the stud spacing to 400mm centers is an option but then as you are using an imperial board you lose the gauge on the end stud for the board overlap, you would have to re gauge to 406.7mm centers to make it work, or just cut all the boards to suit, but then add in the fitting of doors and windows and along with the cripple studs to the openings, you could end with studs closer together, and very little room for insulation and for little gain.
It's left me in a quandary about roofing material
The main factor to account for with regards the roof material choice, is its pitch angle and then how it needs to be fixed (some fixing spacing's are as close as 450mm and can be up to 1200mm) which, will then dictate, along with allowing for the roof load, as in its dead weight, then factor in the snow load, and also the possible effect of wind if in an exposed area, the design of the roof trusses.