I tried to get the X-ICE Snow but it got backordered month after month.
How much are you lifting your foot? I am always touching the accelerator and just adjusting how much I am pushing down to go from acceleration to deceleration.
Are you actually lifting off the pedal? As in there's air between you and the pedal?
I only take my foot off the accelerator pedal when stopped (to rest my foot), in an emergency, or if I am hitting the brake pedal. If you lift, and cause a lot of regeneration it's like hitting the brakes hard, the weight of the car shifts forward and the rear end gets light, lifting, and allowing more fishtailing.
If the rear feels like it is stepping out, or wiggling, you need to stay on the power and counter steer as necessary. I don't mean you have to floor it, but you need to keep positive pressure on the pedal as the motors are powering the wheels to gain grip (especially in a rear-biased system where you need to kick on the front wheels for assistance).
Your S5 was 40/60 Front/Rear, so you should be used to the rear wiggling a little bit when you give it a bit more power. My WRX 50/50 and STI 41/59 wiggled a little, especially off the line in snow.
If it was manual, then you should also be used to engine braking and so you would lift off the gas a lot less. If it was QTIP, then you might be used to lifting to coast, which won't work very well with a Tesla and could be part of the problem in the snow.