Just want to clarify, when you say “fully assembled belt on all accessories in play”, is it just one serpentine belt?
Only reason I ask is that if this is the case, are you rotating the pump the wrong direction relative to the impeller blades?
Got a picture of the belt(s) routing? Also wondering if you have one really long v-belt going around multiple things and it’s just slipping on the water pump pulley and not driving it fast enough because it’s not making enough contact.
Last thought would be casting sand or sediment from hard water inside the block, or as another asked, hard block.
Hope you find your issue!!!