OP, AP lit, AP gov/reg econ, AP physics C, and multivariable calc are fine rigor. The mistake, the “lighter” to worry about, would be easy, more generic classes. But I agree, why AP Span? Put the right effort into the AP lit, do better than a B, if you can.
I don’t see where you tell your grades and scores other than an ACT 36. It will matter how strong your grades are in cores and the classes related to the stem major. And you’ll need the right sorts of ECs, depth and breadth.
Drop down to regular English. Your rigor is just fine. Having physics c and Calc 3 shows a lot of rigor and will be more key for your fields of interest then AP lit. Science /math kids typically are not strong English students anyway (just saying). Enjoy your senior year.
I think there is a big difference between AP Lit and regular lit, not as much difference between AP lit and honors lit. IMO that would take you down a notch when it comes to rigor, but honestly a “B” on an application is not really going to make that much of a difference – and you can do better! Either way it won’t make that much of a difference, but since you have dong AP Lang and you have the option, I would go with AP Lit.