My suggestions for the Select & mask Brush tool
1. Add the Opacity slider.
We can just choose the Hardness, but the Opacity is always 100%.
Not good for precise work as some areas may need a bit of transparency.
2. Add the Blend modes.
Pro editors often use the Soft light & Overlay modes for mask adjustments.
It preserves the darkest/lightest pixels and helps to clean up the mask easier & faster.
P.S. Yep, those manual adjustments can be done outside the Select & mask window, but since there's a Brush tool, why not to have its full functionality?