If it were me, I wouldn't bother trying to machine the miters; I'd just machine it straight and cut those later with a saw. You never get a very good cut routing wood against the grain like that. If your machine is big enough, you can probably cut all 4 pieces at once. In any case, it's good to leave something at the ends to hold onto while you're routing on the middle. As for the slanted face (which I assume is on the underside in your illustration) I'd cut that in a separate 3D machining operation, using a ball-end endmill so as not to get the "stairstepping" you'd get with a flat-end endmill.
Andrew Werby
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