I don’t downshift in slick conditions, no matter the transmission type or which wheels are being driven.
The end effect that you are trying to accomplish all your deceleration with two wheels in 2wd cars, or proportioned to the drivetrain in AWD cars.
The brakes in all cars are balanced to give the most effective bias front to rear in braking (typically 3/4 of the force from the front tires due to weight distribution and force transfer). Downshifting will upset this balance and make the driven wheels more likely to slip.
If anything I’ll shift into neutral to brake in slippery weather.
For the OP, something copied from the SUV vs. minivan thread: You can accelerate, brake, or turn the steering wheel. In slippery weather, only do one of these at a time. This means thinking ahead and finishing your braking before you turn a corner, for example.