Re: Have I overcooked it ?
I used to live on the USA West Coast where summer is always sunny and over 100 degrees F for weeks on end.
I always had the "dash carpet" (fabric dash mat) on my dash, and a shade in my front window to protect the dash from the extreme heat and cracking. You might want to also consider UV-resistent film on your front side windows (and on the windshield if allowed by law in your country area).
Do not listen to the negative comments on this forum, from people who have no clue of why you are doing what you are doing.
If your dash is several pieces, with visible stitching - it is the leather dash. Otherwise, it is a vinyl plastic material.
I would rescue that dash cover, clean it up and put it back. It actually looked like a decent fitting piece, unlike many. Best luck.