I updated, despite my intentions not to. I have exactly the same issue with the 9.7 that you did-- not quite enough screen real estate but the 12.9 is just too big. I absolutely love the 10.5, and it does go some of the way toward alleviating the cramped space of the 9.7".i really love my iPad Pro 9.7 (with brydge keyboard) but the screen size has always been the one glaring limitation for me. The 12 inch iPad is really too big for my needs but the 9.7 has felt small too me. Has anyone made this switch? Is it silly too even contemplate?
Just remember that it's less than in inch in the diagonal, which really isn't that much more space in width or height. I think part of the reason it feels better isn't the actual size of the display, but the perceived size of it. I don't know if it's the narrower bezels or a change to the display stack but I feel like the pencil is much closer to the pixels, and the parallax it un-noticable.
There is no perfect answer to the screen size vs. portability problem though. Apple bought some ground by shrinking the bezel but mostly portable and light means less screen. Just how it is.
So I'm really happy with mine, it's better in ways I can't really articulate, but I also had the advantage that I had someone to pass my 9.7 too, so there wasn't really a loss on the upgrade.