Just a suggestion from how I deal with rounding corners. If I have a rectangle slot like you have here, and I want the ends to be rounded, rather a full rounded end or just slightly the corner of each part of the rectangle, then I draw in a circle the size of the radius of the circle/ roundness of what I want. Then I make sure that the whole area is highlighted, then click trim vectors. Then I remove the sharp corner and the inner circle that is not desired. Then I click on join vectors and the corner will now be a rounded corner in a full line.
Just remember, the tool diameter and radius cannot exceed the radius you are cutting or the cut will not come out the way you intended for it to be, or the contour toolpath will not even give you a cut within that slot.