Window head and reveals are much better dabbed Then wet plastered up to them Then a skim bead on the edge Far easier to get a nice even margin and line
Loads of answers So probably repeating advise
As a tiling contractor The problem is in the first sentence 6 mil ply is your problem It should have been at least 12.5 The matting will make little difference
I would have perhaps started with your location and budget
I found that using 70 mil insulated plasterboard dabbed directly onto the exterior walls has worked a treat on our previous build Hopefully does the same on our current build
With three vaulted ceiling’s I’ve used 120 insulation between the rafters and taped them in multi foil
No
not really My wife and myself fitted the three sets of slider on the Saturday morning and I rendered the reveals in the afternoon It took longer to sheet the new doors up