Simon - I believe you've been doing this with a "one-shot" approach. Have you ever considered seeing what the results are with a few more prompts? Maybe 1,2,3 adjustments?

Something like the astra MAX is pretty darn good - but something is up with the right wing and the right foot (flipper?)

I bet each of these could be modified to be significantly better with 1 or 2 "rounds" of adjustments. (Others not so much).

Obviously, not as deterministic as your single prompt approach, but something I just thought of while thinking about the price (Because wow! For some of these I'd expect a usable SVG after that much).