my tub and shower combo will look just like this, but the tub (Kohler underscore) is 21" high. Would this be too high to have extend into the shower for a usable bench?

As long as the tub will be installed above the decking surface, it should not be an issue. A standard bench is 18" high, in the image shown, the tub deck is 23 1/2" high to accommodate the bath installed. To reduce the overall height of the bench, assuming you are on a raised foundation, you can sink the bath into the sub-floor to create the perfect bench height. The Bay Area Remodel Bath was on a second floor....so a higher deck was the only option. Good luck with your bath planning!