I was imagining tapered insulation. They sell pre-sloped panels that are tapered at 1/4" and 1/8" per foot. You stack them with flat panels and can slope as long as you want.
Here's an example.... https://www.gaf.com/en-us/products/energyguard-tapered-polylso-foam-roof-insulation
I've only used it on bigger jobs, so i am not sure how cost effective it is, but it definitely could simplify framing.
Let me know what you think.