Haste makes waste, measure twice and cut once. It's one of the hardest lessons to properly internalise - bodges and firmware changes can be hugely time-consuming, so it nearly always makes sense to absolutely nail your prototype before moving to production.
Congrats on a cool project.
One/one-hundred-off prototypes are expensive but nothing beats the information gained from soldering a board, plugging in connectors, and programming chips directly. You'll do these steps at the end of a waterfall process anyways, and I'd say they're more expensive to alter then. When a problem is found, deciding to bodge wire two pads is the high-cost, low-risk solution to problems found late in the game. Discovering the problem early, fixing it in the schematic, and testing during your next prototype cycle is cheaper during production and risk is mitigated because you're still going to be testing before you ship.
Of course, this is all very much propaganda, and not all shops would follow this idealistic scenario, but I don't think hardware is any more waterfall than anything else.