“Service Brake Assist” and squishy pedal

[Matt_7]

2016 Sierra SLE 4x4 160’000km

 

Looking for some help with my brake mystery. 
First time I noticed the issue was this spring driving through heavy rain. I got a pop up on the dash with a “service brake assist” warning and a squishy, vibrating brake pedal. No ABS or CEL to accompany this. This. Continued intermittently, sometime starting right from the driveway, sometimes mid trip. After doing a bunch of research, I initially replaced the booster as it is apparently a common failure, no change. I then thought maybe wheel speed sensors. I initially tried testing them against each other for a short or inconsistent ohm reading - no luck there. So I started replacing them. After replacing the front passenger sensor first and unplugging the battery to reset any codes - brakes were good for a couple months…. Until yesterday. So my theory is resetting the system was possibly a temporary fix. 
 

I’ve read lots about the notorious brake system recall - doesn’t seem to match my symptoms. My pedal gets soft instead of hard, and I have no issues stopping. Truck almost stops too well when it’s acting up. After more reading about the brake assist function - I’m thinking maybe this system is over-activating for some reason. Anyone else have this issue, or have any idea of where to start? I’m out of ideas and not excited about the million dollar bill to be expected from a dealership.