I got the Combo touch 12.9 recently for my M1 (I already had the 11" one) and honestly it takes a lot more space, making the iPad barely usable on your lap, to the point that I am using the combo 11" keyboard with the 12.9 case (it works). There is a lot of wasted space since the keyboard is essentially the same size, but the screen is larger and has a taller aspect ratio.
I took a picture to compare:
View attachment 2523546
Maybe they improved things on the M4, by using that space for the trackpad, but this does not improve the lappability since the screen is the same...