AMG GT Brake Pedal Feels Squishy

I have a similar AMG GT. Strange brake pedal feel can indicate issues. If the strange feedback is related to undervoltage the car's computer should generate an error. Check if the ABS or ESP lights are on. These systems rely on hydraulic brakes and the brake calipers. Have you checked the condition of the brake pads and brake shoes?

No warning lights are on, which is even more confusing! I haven't actually checked the brake pads/shoes myself yet. Is it likely that undervoltage could cause that kind of brake issue even without any warning lights lighting up on the dash?

Undervoltage can cause unpredictable issues with the brake system even without warning lights. It's difficult to diagnose remotely. Best to get it checked by a workshop, they might be able to find the issue. Replacing worn brake pads on time is always a good idea.

I took it to the workshop, and you were totally right! It was indeed undervoltage messing with the system plus worn brake pads. They sorted it out, new pads all around, and the electrical issue is gone. Cost me about 240€, but totally worth it to have my brakes back to normal!