Much thanks guys. Like @Vince.Fab said, increasing cutter engagement helped with stability greatly. The chatter completely disappeared when I went back to a shallow wide cut, I guess I should just stick with what works and no try to optimize too much
I ended up running the tool path with my “Best Performing” settings above (0.9mm DOC, 4mm WOC), but with the adaptive tool path keeping a constant load in the corners (vs. the 2d Pocket I used previously) I was able to run at 150% feed rate!
I also tried out a radial finishing pass which ended up looking pretty good. Now I just wish I’d run it on the axial surfaces as well.
Here it is installed in the car. This particular bracket is to adapt a cheap off the shelf EGR linear position sensor from a Ford 302 to measure turbo wastegate actuator position on my track car. Please forgive the dirty engine bay.