I typically prefer to use new subject line for each email. That way, with a new subject line, you can test open rates better since most people open on a new subject line, but if they missed the first one, their inbox sort of collapses one against the other. So if you thread it, and they don’t connect with the first subject line/value proposition, the next one gives you a chance to get back in there. Readers decide which emails to read based on Subject Line so I think the threading on a colder outreach is risky.