Hi,
Be cautious of any novel wall assemblies. Maybe novel in your area? In which case, make sure your contractor is comfortable with what you’re proposing.
Trusses are often used above 150 mm because, as you’ve said, fixing rigid insulation can become tricky beyond that. Rigid insulation systems, such as Gutex, can go up to 200mm. The insulation supplier/manufactures will specify everything, down to the very screw patterns and angles, to get warranty. Not sure what they do with very uneven walls.
Wall truss assemblies typically use blow in insulation because the exterior sheathing allows for consistent packing factors and densities. Vertical cavities are better than sloped and horizontal ones for this. You don’t need to worry so much about uneven masonry, as the trusses can be shimmed to be flat and plumb.
*edit, I just got to the bottom of the article you referenced, they did not go for a sheathing system, but a breather membrane on the outside, after several overlapping layers of insulation and bracing/furring strips. Looks like a very labour-intensive way to do it imo. They had a lot of money.
However, it is better practice to have the air control layer on the masonry side, as well as the sheathings water and vapour control layer.
The usual suspects; punched opening, vertices, edges, and services; are where all the detail and the skill is needed. I’m confident with the theory, but don’t ask me about doing this in practice, let alone doing it productively.
Windows in particular. “Outie” windows, with thermal bridging detailing around the casing and frame, along with flashing and sealing details, are complex.
Cellulose will likely be your most environmentally friendly option. Consider doing it alongside blowing in insulation elsewhere, such as your loft, because it’s the call-out fee and equipment that cost a lot, not necessarily the material.
You could go over 200mm without too much extra cost, if the transition to your roof system and soffits allows for the build up and convenient continuity of control layers.
Ultra-high levels of insulation can lead to hydrothermal issues on the inside of the exterior sheathing in cold, wet, climates. You’ll need a back ventilated drainage plane behind your cladding system, 10mm or so for exterior renders, 20+mm for timber cladding (check fire codes). Plywood is better than OSB with moisture, and some plywood is better than others. I think 10-20perm is recommended. Interior vapour barriers are highly suspect in these situations. Be sure you know what you’re doing at the very high end.
My personal view is that 150mm rigid insulation systems, that are proven, are fine for retrofits. Save the money for the windows and doors, or the mechanical systems such as underfloor heating and rigid low static duct systems.