I think I can help you here TJ.
A few years back I built something almost akin to your plan due to wanting it to be moveable , and it's survived the gales very well indeed when other plot neighbour'shop bought badly designed greenhouses and polytunnels have been wrecked. It was originally 8x6 but I have now extended it to 12x6ft.
I used 50x50 tannilised timber for walls and roof (19x38 roofing batten is a bit small in my mind), drilled and screwed with appropriate 3" screws.
Being a builder/carpenter, I added a small 6mm ply triangular gusset to each roof span apex, and anchored the whole greenhouse by screwing the structure to 4 no. 3x3 old posts concreted into the soil as you would fence posts. I made up the roof frames at home where I could cut them to length and the appropriate mitres and gussets required. Woodwork was spaced at roughly 620mm centres to fit 1200 high x 600mm wide 4mm uv treated greenhouse polycarbonate sheets..... very strong. A good price at nick-gray plastics at his website or his ebay shop.... 2 x 10 sheets of 1200 x 600 will be enough for 8x6ft structure apart from one end wall, for which you could use something else. Those packs of 10 are £37.50 each... a good price. I've bought several things off him for other building projects and he's a good efficient business.
The polycarbonate sheets help strengthen the building as they act as screwed braces. I also attached one diagonal 50x50 brace to opposite corners to add further strength. Also, the pitch of the roof was determined by the 1200mm length of the sheets with a slight overhang for guttering purposes. All four original "roof frames" were tied together with one internal 50x50 ridge beam attatched to the uniform sized gussets.
It stood me in at £130 all in.... made the door out of 50x50 as well and just hinged and hooked it.
The polycarbonate acts very well as an insulator, far better than glass, lets in 90odd% light, is treated and lasts for donkey's ears..It also doesn't get tooooo hot in the Summer, like a polytunnel can. I taped the ridge joint with aluminium tape, but any strong tape will do. Sheets were just screwed to timber using 30mm dry wall screws, mostly with little washers to spread the load of the screw, although not really necessary.Every 200mm screws on roof sheets have created a waterproof seal to the timbers.... I haven't needed to silicone it at all, and of course, I had in mind for it to be moveable.
Being reasonably tall myself, I wanted a bit of height too, so the actual side walls floor to eaves height is 1700, so I screwed a 5o0mm base dwarf wall around the bottom of the posts with slats acquired around our allotment site. The pitched roof gives a good 7 to 8ft height at the ridge.
Here's a picture from yesterday with early peas waiting to spring forth