Generally, when machining wood, the governing factor is holding the stuff. Without knowing how you are holding your material to the bed it's difficult to advise. Usually I just depend on the vacuum pump, so with a carbide 2 flute 6mm cutter I would not cut more than 3mm deep per pass. Revs would be circa 22,000 - 24000, and max speed 4m/min. The machine could cut deeper per pass, but that would mean taking extra time/effort to secure it to the bed first, so I take the easy way out.
If your software cannot step-down per pass, a possible solution could be to duplicate the contours with each consecutive contour to an increased depth. For a good edge at 18mm, I would step down in 3mm intervals for 17mm but with increased offset (0.2mm - 0.5mm.) Final cut would be to 18mm deep at the correct offset. I never use tabs either, as they seem to be detected in the final passes.