What would be a good way to navigate this email conversation with the co-founder, so that I align a potential job there with my career goal?
Something like "Thank you, but I'm not looking for a QA position. If you have a software engineering slot, I'd love to demonstrate why I would be a great fit."
Unless you actually want to be a professional QAer, don't take a QA position. That wouldn't be good for your career, and it wouldn't be good for the QA Team. I speak from over 25 years of leading QA teams.