Here’'s baseplate recommendation that helped with back foot burning for a newer parawinger.
Otherwise, if its a foot position issue then Gwen has a good explanation of standing on the sweet spot and foot position relative to your hips. Counter-intuitively moving your back foot further can make this problem worse… instead move your front foot further back and maybe even move your back foot further forward to narrow your stance over the sweet spot. This made the difference for me dropping down in foil size to maximize the amount of glide and low-end stall speed. As a 195cm 100kg rider this means i’m mostly riding hip width, to keep up with small waves or wind swell. It could be helpful to see your stance while riding to see where your hips are. Straight front leg means weight too far back, and burning out that back leg. The smaller the foil the smaller the sweet spot, so tuning mast position on your 980s might be easier to spot the issue. When pumping this translates to a solid pressure in your front foot like a platform that you are able to hop off of, rather than a squishy seesaw you are going up and down on.
good luck!