I'd love to add a few branches to fix the gaps 2/3rds the way up the trunk. Maybe time to grow some cuttings for approach grafting

Thanks, still living with the consequences of my mistakes of a decade ago: not sorting out the roots; leaving a bar branch; chopping out one too many branches. But that's how one learns.
Probably not much. There’s two main reasons: firstly the tree is no longer budding back to bases of the branches. Again, a mistake from many years ago when I didn’t keep the branches short enough. Compounding that is the difficulty of getting a believable foliage pad given the way the foliage grows (I’ll have to take a photo of what I’m talking about).
Unfortunately I can't take 3D photos.