I always use a 315° angle which makes the first cut a hole. No strain on the bit, every cut is a step-over cut. My cut are usually with a 3/64" bit up to 0.20" deep single pass and haven't broken a bit yet while cutting.
Its difficult to evaluate without seeing the cnc in action, but, we can all make educated guesses
Did you create a border vector for your lithophane or just using the photo?
I always use a border, often not a rectangle, stars, hearts, etc.; just select the border vector then create the finish toolpath [and roughing if you use one].
Good Luck! Dale