Drawing done in Bluebeam (ie. sketchy!)

I hope those are stainless brackets (as opposed to galv steel) to minimise the bridging. Compacfoam would have a been a more thermally efficient way to provide a mount: https://www.youtube.com/watch?v=5sSKsKB9doc
Green Building Store can send you some next day delivery :-) The main catch is that it's fairly expensive.

You don't say what the surface will be. Presumably you'll be rendering over the insulation, hence the query about space for the beading? I take it you don't have some beading to measure and check how much space is needed? The inner bead needs 15-20mm and the corner bead another 20mm. I guess you can overlap those so 25mm would work, but 40+ will be more comfortable.

I'd put some 25mm pieces of insulation over the frame so most of it is covered.

You don't want to push the window too far back into the brickwork, so maybe more than 80mm insulation would be a good idea? Why not 100 or 150? Decent U-value, plenty of reveal depth, metal brackets not bridging insulation.

Have you worked out the other details? i.e. got cills with the right render-ends and maybe some foam jointing tape? And something to waterproof the bottom of the window (under the cill) convincingly? And worked out how the window airtightness is working? I used illbruck airtightness foam and tescon tape. (and fancy ampacoll tape under the cill - that may well be overkill if not using woodfibre).