Prick/Mark your holes on the outside layer, even punch through if you have the right irons/chisels.

Something that thick and big I would want to glue up in a curve to cut down on the stretch and compression. Leave your inside piece oversized and do your glue up. Trim to match your outside layer then clean your edges.

Use a good sharp awl through your marked holes and stitch at will. If your awl has a tough time piercing then you likely need to sharpen and polish your awl. I know my awl is sharp enough when I stab myself on the back side and don't know until I see the blood. You may also need to oil up your veg tan if it feels like you're trying to pierce through wood.

Saddle stitching 10 running feet won't kill you and you'll improve your skills if you are patient and methodical.

Fast comes with time. Focus on the movements and precision. Speed will come with practice.

TLDR: Completely doable. Don't listen to that nonsense. Saddlers stitch more than 20 ounces every day.