I know you said you looked, but it still sounds like floor mats or pedal position.
Pedal mount loose?
If you have the adjustable ones they could be adjusted to the top for diagnosis.
Did you check the fuel filter %.
The symptom you are describing is exactly how my old LLY behaved when it needed a fuel filter. (fortunately on that one, they were easy to change on the side of the road)
2018 L5P Sierra 4x4 SB Denali
Sold 2011 LML Sierra 4x4 CCSB SLT
Sold 2010 LMM Sierra 4x4 CCSB SLT
Sold 2004.5 LLY Sierra 4x4 ECLB SLE