...In fact, TAS never gave us the standard shuttlecraft from TOS...
Actually if you take a peek at the panning shot of the hangar bay in TAS' "Mudd's Passion" episode on the left side you can see a design that, while poorly drawn, looks roughly like the familiar TOS shuttlecraft. And you can note that it's shown way oversized along with the other designs. So that there gives me another rationalization for scaling down the TAS designs.
In that panning shot there's another shuttle design on the right at the front of the row. It has an unusual ovoid shape to it and we never see anything of it in the series but that glimpse in that one shot. I'm thinking of trying to work something out from that.