I reckon your programs are binning it off is because you are trying to use a tool that is the same diameter. (10mm circle - 10mm ball end)
Unless you do it as a pure slot with 0 tolerances it likely won't go like this.
You could maybe 2D slot it with ramping and no return flattening path. Using multiple depths set to the finished depth of the deepest area. Enter at the shallow end and play with the ramping angle until the angle matches the slope. (It's a bit of a fudge).
Other option. Try a smaller tool using a 3D toolpath and see what that does.