You could as has been stated put two switches in, one flush, one add
water, or one to do both.
However the smart flush control also has that, the two bottom buttons are manual flush and manual add
water, so other than saving a few $ you haven’t done anything.
Plus the smart flush is programmable, how much water, flush delay etc.
DIY isn’t, and depending on line length etc, being able to program how much water etc is worth having.
It has a water saver that flushes once, and the regular mode flushes three times, not twice.
Gut feeling is that the three flush is worth it, to clean out the lines and hopefully make them last a lot longer before they permeate and begin to stink.