Helpful post, thanks!
I think the exercise you recommended is exactly the one I've been working on yesterday and today. Started practicing in C (relative A minor), moved up to D, then E etc as an exercise on those top two strings to get a feel for the changing shapes of those dyads moving up the fretboard to play the major scale. So, for the C major example, playing only the root and third notes on the E and B strings up the fretboard would give you the full harmonized major scale of C > D minor > E minor > F > G > A minor > B > C.
This has been one of those small game-changer light bulb moments for me because I only played the double stops that were easy and comfortable and sounded good without any consideration of the intervals or what it was I was playing. All based in the boxy shapes. My crutch is typically playing linear double stops built from the pentatonic boxes like this....
I'll also focus on the D and G strings for pattern 2 and play those together as well. (Are all these perfect fourths?)
What I'm not doing enough of is string skipping or hitting on the 1s and 3s, it's all just basically playing chord chunks laid out right next to each other because they're easy to find and play together. The problem is that although my playing sounds like I know how to navigate the fretboard in a key and play the pentatonics and arpeggios and the full major/minor scale, my double stops are repetitive and formulaic. They fit structurally and sound good, but.....kinda meh after a while?
Is the issue playing the same interval without changing it up enough? Seems like all I'm doing when playing those double stops is taking little pieces of the pentatonic boxes, and I'm nor really sure that's saying much wrt playing guitar.
At least this exercise is forcing me to expand that comfort zone.