Try the drill on the drive shaft again but this time block the flow with you finger looking for water squirting out anywhere. Then slip your finger off and watch how well the flow recovers.
Thanks Tubs.
When I put my finger on the hole, pressure builds up immediately and the water forces itself past my finger. It hit the ceiling in my workshop. So I’d say that has both plenty of volume and pressure. It seems to confirm the problem is higher up.