I’m not a Harvard level applicant, but at my school we rarely have every had students get in in SCEA. It seems like most of them get in through RD (2-3 each year), as the EA pool truly seems to be self-selective. The one that got in last year through SCEA was as close to a shoo-in as you could get (lots of national/a few international awards, one of the highest GPAs the schools every had, perfect test scores, etc), so it’s an unbelievably competitive group. Plus, the acceptance rate is for sure higher than it seems because that’s how all the athletes get in.
It really seems like Harvard’s SCEA is almost a waste of a potential ED that you could use at somewhere like Northwestern, UChicago, Duke, etc. If I were you, I’d ED somewhere and do EA to MIT or CalTech. UChicago also has an EA but i don’t think it helps much. I think you could do all 4 of them if you want (ED somewhere, plus EA to MIT, CalTech, and UChicago) but that might be a bit ambitious.