I think this is a cool tank.
I have almost the same set up as you. 125 gallon buffer, 35 gallon chem tank. Right now I have a 25 gallon tank as a down stream tank and for me it is a little too much.
16 gallons would be just about right to wash 2 houses a day with a few extras (decks, fences, etc.)
Normally I am washing a house then running to do some windows, I don’t straight wash all day.
Also with the 35 gallon plumbed to a hose and ball valve I can make different mixes if I need too.
This way you could also draw off the 35 gallon tank with a 12 volt pump to mixing valves and that would give you the ability to have over 100 gal of roof mix at your disposal.
Just another idea.