Daox, in the highlighted area, I would try first using some of the foam channels to get air from the new soffit vents into the attic area. Then fill the rest of the volume between the channels and the interior with spray-in foam (probably need an insulation contractor to come spray it in, but it only takes a few minutes once they get going). That'll have the added benefit of sealing all the air leaks. This can be done from the inside without having to compromise the roof, though you do have to remove the ceiling drywall over the highlighted area temporarily.
If you want added insulation in the area, then at the same time you could add furring strips to the bottom of the rafters to give you extra space, then slip in more insulation before reinstalling the drywall. Of course this "lowers" the ceiling in that area, so it's a bit of a trade-off.
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