I'm still sort of opposed to making the Copernicus into a small, shipboard auxiliary, when she could more directly represent the "runabout" school of thought and be distinguished by the factors we already witnessed in the episode: a rather cavernous interior for long endurance, an overall size worthy of a long range vessel, and a complex stern structure that features exotic doorway arrangements and quite possibly also hides an elevator or a stair of some sort to allow our heroes to descend to the surface.
But if it's going to be a shipboard craft, the Copernicus is probably best described exactly as you make it look: true to the general shape and feel of the cartoon art, but with the rounded edges and solid shapes familiar from the TOS prop. An impulse engine integrated into the aft hatch would probably be fine, considering the TNG "postcedent"...
As far as I'm concerned what I'm drawing is not a vehicle for assignment to a starship as part of its regular shuttlecraft complement. Neither are the other TAS designs. They're based at starbases and outposts and can be loaned out for specific missions. As compact as I can try to make them so they can at least be parked temporarily on a starship flight deck they're still just too big to be part of the regular complement.
While I am trying to add some detail to make the design look more complete I cannot integrate an impulse engine setup (that I care for) without starting to really change the overall look of the craft. I've tried...numerous times.
Oddly I'm not that crazy about these designs. I think I could do better with a fresh design. But the point is to rationalize what we saw onscreen rather than completely re-imagining them. So I'm doing the best I can with what we were given onscreen.