When you have oversupply & overactive letdown very often your baby can nurse super fast and be plenty full. My baby nursed just a few minutes on one breast and would be finished, and he got really fat (rolls over rolls over rolls style fat). I know it is unnerving at first, but if weight gain and diaper output are good then you need to trust that your baby is getting enough from these short nursing sessions. You can keep an eye on diapers and offer to nurse more frequently if they drop off, but it really doesn't sound like that is and issue at all.
with PP, the amounts that you are pumping indicate oversupply. Stop pumping. That will only make things worse. Pumping aggravates oversupply and overactive letdown.
Try nursing in a reclined position. You don't need to know when your letdown occurs for that to work. I think it is potentially more effective than most of the other tricks out there for overactive letdown too. I didn't feel a letdown either early on, but I definitely had issues due to overactive letdown.