Why not just cut out a rectangle of plasterboard between the joists that you can climb through ?
Much safer and easier than climbing ladders and lifting tiles and not much work to fix afterwards.
This is what I did when insulating the "slopey bits" of the ceilings in my house. However due to the inconsistent width from top to bottom I opted to use fibre glass at 65mm thick to maintain a 50mm air gap rather than kingspan type product. Worked a treat and made a big difference. I also insulated the vertical plasterboard walls at the same time with 75mm cavity wall pads and a small bit of ceiling from the rooms below. I finished it off with an extra 40mm of insulation on the sides of the dormer windows, that being the maximum I could do due to the window.