You are right - this is a very tough schedule, but there are others who are taking similar schedules, with similar rigor, yet are taking five or even six AP classes in senior year.
I would consider the quality of the teacher. If the APUSH teacher you get is fantastic, it might be a very enriching educational experience, certainly of value in today’s political climate. If the only possible teachers for APUSH in your school are horrible, then I would go with regular US history. I don’t think that, as a STEM applicant, top STEM schools are going to look at your class schedule and say, “Well, that’s all very nice, but why didn’t he/she take APUSH also?”
You could always register for APUSH, and then drop down early in the semester to regular level, if you don’t like the teacher or the class, or if it’s immediately evident that the load is too much. You don’t have to take the AP exam, whether or not you stay in APUSH.