That's fair. I think that running product takes a lot of customer empathy and you're right, the founding engineer can definitely be the customer advocate. One worry I have would be whether someone in that role would be too interested in the 'how' of building, rather than the 'what' and the 'why' which are so important for product. Hope it works out for you!

Another role I left off was CEO, but that one seems less likely than product.