Thanks for chiming in!
I am using kiri:moto to toolpath, and as far as I can tell it doesn’t have spiral cut. I had thought that “ease down” was ramping in but I think not. [Update: it is intended to be ramp in but is not fully implemented at this time.]
My first try was to clear the voids with a 6mm ball end (24000 RPM, 1600mm/minute) and then take another pass with a 4mm ball end (24000 RPM, 1400 mm/minute) intending to get the corners of the pockets (as shown above the outline in the picture) but it was starting to chatter going into those pockets, so I turned the feed way down to 20% so 280 mm/minute. That was when it started wandering while cutting the outside and I hit pause.
The sides have a 2.2° draft angle, so a ball mill seems necessary to avoid a stepped finish. But now that I understand (I think) why the carbide bit exploded, maybe I should get a 4mm 2-flute flat endmill to rough, and use a 4mm ball mill to finish, so I don’t have those pockets. With that I might even be able to rescue this part. (The wandering section isn’t a big deal.)