Update:
Took the pump apart and replaced the impeller and all the seals/gaskets. Reassembled it and turned it on and it seems to be running fine.
However, the same issue is still persisting. Grabbing the PVC pipes, the water from the pump to the filter is pushing hard, like normal. However, the return from the filter going to my heater doesn’t feel like water is flowing at all, at least not with any notable pressure.
So I opened the filter to take a look and noticed a crack/hole in the top manifold. I also noticed that the filter grids were only coated with DE on the bottom half. Obviously going to be replacing the manifold, but could that be the source of the problem?