It seems to me that this is the biggest question you are asking in your post (which school to choose for REA). I can’t chance you for these schools, but in general, any time a student is clear that they prefer one school over another, I think it’s better to try to maximize chances for the preferred school, rather than the second choice school.
So I would suggest that you talk to your counselor about whether they think applying REA vs RD would benefit you in Harvard admissions. They know you and your school much better than we do, so they should have some insight here. I do not know if REA at Harvard would be likely to have any benefit, but if they think it would based on their experience, I’d go for that.