You set the Units correctly to prevent displaying them in the dimensions. There’s no where else to make that setting anyway.
As for making the dimension in red, what’s the problem. You can do it like putting in any other dimensions. The proper method would be to run the lower extension line from the outer corner, though.
It was taking the diagonal measure at first place, but depending on the direction I move the cursor I can get what I was looking for, the same you did. I’m so dumb