An update simply to log this for future searchers:
I had a suspicion that the front left wheel was bent and took the wheel in today to have that checked out. Low and behold, there is a slight lateral wobble to it - maybe 3 to 5 mm. From what I understand, a wobble or bent wheel will cause the ABS/Speed sensor in the hub to misread the wheel speed as it rotates around the hub. This in turn tells the ABS computer to do its thing for that caliper OR that end of the vehicle. Being that this pulsating thing was a front end issue, the fact that the wheel is bent makes sense.
The snow wheel-and-tire setup has been mounted for a few days now and the problem has all but gone away. In the near future, driving around with INPA running will tell me in real-time what the ABS wheel sensors are measuring, loosely utilizing points from this thread here:
https://xoutpost.com/bmw-sav-forums/...-question.html . From what I understand, the sensors are crazy sensitive and with minute adjustments using data from INPA anyone can get the ABS system back in check AS WELL AS saving time, effort and cash over replacing the sensor(s) outright.