Add a blur brush to blur contour after masking the foreground image
Instead of masking it with a smoother brush
Brendan O'Regan commented
A focus/ blur and sharpen tool to seamlessly match & blend a crisp, high-quality source foreground image with an out-of-focus background layer or vice-versa.
An automatic select option that blurs or sharpens image components by a threshold percentage feature, similar to the magic eraser's abilities, would be and ideal co-function.
But primarily it would be a blur/ sharpen brush tool to manually work on specific areas with a controllable degree of focus.