For info, I’ve done reasonable isolation routing using 0.3mm dia end mills. Those have a cutter length about three mm and will snap if you look at them wrong. But great cut compared to v cutters, but together with their fragility are wickedly expensive. To OP, dial out all your other problems before entering the world of exotic cutters.

It’s this experience that resulted in the question whether the image of the snapped cutter is an end mill or a PCB drill. The cutter length looks to be too long for a cutter suited to PCB, but your experience may vary