Evaluating red flags in potential advisors' scientific production and impact, past students, research interests and personality
Well, first of all, this is a master's thesis. I would argue that it doesn't really matter that much, because you will at most be working with this person for a year or so and then leaving to go somewhere else.

Given what you've written here, I wonder why this is even a question for you. You know Prof H and have taken courses with him and done well. You know that he gives good mentorship and feedback. You know that his former students end up very successful. You know he's productive and well-known, which could be helpful for getting into a PhD program. The only question here is does he have enough subfield knowledge in your area to advise your thesis?

Also, if he has 30 years of research and is a household name, how is his work not impactful in terms of citations? You don't become a household name as a scientist unless you've written some papers with impact.

Prof K might not be a bad choice either. The fact that he has had no doctoral students is irrelevant because you're writing a master's thesis. But why go for an unknown if you already know Prof H and know that he's a great advisor?