Dibond can be cut in various ways but the bits that i like are 2 -3 flute straight ones or 2 flute fishtail 30degree spiral upcut which leave a mirror finish, and the bits originally meant to cut Dibond are single flute as pointed earlier. I know of only 2 ways to cut it properly:
-vacuum bed
-stick ultra thin strong double sided tape at least at the important points. Not the one they sell at stores. Best tape for purpose is photographic double sided thin tape used to bond methacrylate to photography. Its sold on rolls and 50m is 150-200 euro, but if you ask some sign shop for a meter it would last you months. Final pass should be no more than 0.2mm deep and slow as the tape is a bitch. So tape should be applied on small squares of 2-3cm here and there. Stick square bigger than 10x10cm and you will never unglue it, its so strong

Possibly you problems with finish are from sheet flapping. With a sharp bit and air blown at the bit , when well fixed, you can cut it with speed of spindle for wood and say 1/3rd down the speed, at one pass/when vacuum table/.