I was just figuring this out this week actually.
I believe the default zoom is clicking the left control stick, but the weird thing about it is, it also takes you to your capital county simultaneously with zooming in.
That's the only default binding I left in place with regard to zoom.
The other stuff I did was set L2 to reverse scroll wheel. So I would click the left stick to zoom in, and squeeze L2 a couple times to zoom back out.
Then I have another totally separate way to zoom in and out. I set one of the touchpads (I think the left one) so that clockwise and counterclockwise finger tracing on the pad would scroll in and out. So that's a nice one way to do it if I don't also want to be snapped back to my capital. (it's also the only way to scroll through a pop-up window with a scroll bar without actually clicking on and dragging the scroll bar.)
However I am having problems with the resolution. The profile rundown on the left side of the screen sort of cuts off the portraits of your parents, grandparents, and children since it's squeezed in there so much. Another actually show-stopping problem is that I can't declare war after selecting a CB because the actual button is hidden behind the game speed GUI, and I can't move the war dec window away from the right side of the screen. Not sure what to do about that so I can't really play it on the Steam Deck in the meantime.
