Don't overthink it. Set the waterheater back far enough that a plumber can get in there to service it or change it- that is how it looks in the picture. There doesn't need to be any kind of ladder. Buy a good quality step stool/small ladder and leave it inside. The "door can be a simple plywood panel or custom sized flat panel door. I would put it in without trim and paint it the same color as the walls so it just blends in.

Good luck!

Please check requirements for spray foam. I was told that I needed to cover all foamed surfaces with a wall surface or paint the foam with a very expensive type of paint designed for that use. In case of fire, the foam is toxic.

It will all be dry walled.

And insulated door? Attaching door to unconditioned space.