A company in Milford, PA , Color Plus, makes a flexible crack filler that is similar to spackle. It is made for filling cracks in leather seats. It works well but the only drawback is that it only comes in the color of white which means that the dash needs to be dyed after the cracks have been filled. You might want to contact them (they have a web site) about making a black crack filler(mixing their dye with the crack filler before application).

If you decide to replace the dash, let me know as I have both a black and blue crack free W123 dash (300d, 300cd, 300td, 280ce) for sale and I can also provide a set of written instructions on how to remove and install a W123 dash.