This isn't super helpful, but one that I was working on had this same issue. I left it on in its boot loop while taking it apart to diagnose the issue, and it snapped out of the loop as soon as I took out the topmost screw of the plate holding down the battery connector.
Again, it's not super helpful but it leads me to believe there may have been some sort of debris near the battery connector causing a short or something. Be sure to clean off the motherboard in that area and the area near the flexes for the display connectors.