Exactly my situation, too. NRA Sharpshooter Bar 1. It's not me, shooting low. The red dot proves it.
Look at my earlier post. I think Ameriglo might have a rear sight that will work with the front.
Specialty Rear Sight for Smith & Wesson®M&P 2.0 CORE Dovetail.
Part # SW-2404 | Black .492"H .150" Square Notch - $29
I would link it here, but forum does not allow.
BTW, something happened with the S&W move from Mass, to Tenn. I think they lost all their experienced gunsmiths.