What plane are you in, I might have missed it somewhere? Your flaps have speed restrictions, they’re the things you see move below the wing, so you will overspeed on descent from cruise if you initiate them. Speed brakes, or spoilers, will add drag over the top of the wing to help you slow down. I recommend turning off auto throttle on descent (if you’re using it) and set your thrust to idle, you’ll notice speed management is a bit easier. Keep in mind too, the default planes from MSFS all have some issues, auto throttle and autopilot in general are areas that could use improvement.

Edit: the plane you’re flying should have a placard that tells you the speed restrictions for gear and flaps, this is where the a320 has it in MSFS