Actually, the blending mode that gets used to blend dabs with the “wash” painting mode can also be used as brush blending mode.
It is called “Alpha Darken”.
If you set your brush opacity to 50% (or 51% like in your screenshot), you can never generate more opaque pixels than this, you can however still manipulate the color of all image areas, even those already more opaque than your brush opacity.
However as brush blending mode you can’t control the opacity cap with e.g. pressure etc, it’s always what the opacity slider is set to, so you don’t get quite the control as within a single brush stroke where each dab’s opacity is detrmined from sensors.