back in the day... they used to have those vinyl repair kits and you would mix the 2 substances, smear it in and it would fill the crack... so what you need is a type of putty since the dash is mostly plastic, you might want to try a hobby shop and get some putty from them---they also have the paint to cover it just my thought
Since you plan on painting it you an just use some 5min epoxy to fill cracks (like around the speaker openings) and to hold loose bits in place (like below the side vents) wait 24 hours and file down the high spots before painting. If it's real bad I have used 5 min epoxy to hold it all together and then follow it up with "kitty hair" it's like bondo but has fiberglass strands in it.
I've been thinking of going a 2 min two-part epoxy route... I don't want something rock hard though, a little give with maximum adhesion would be where I'd want to go with it.
I guess I'll have to die grind the cracks open enough to get a good amount in to shape and then flatten it. part of my dash was repaired with what looks like resin and saw dust. on the bottom of one of the side vents. I could do that, its held in pretty well.
I sanded and filled the ridges in the top portion, and then covered the whole top part in black vinyl. I used a piece of 1/4 inch aluminum fuel line in the groove between the upper and lower portions of the dash to hold the lower edge of the vinyl down.