Hi, Dav.
Seems like you have a few amps passing through each of your USB-C cables, which it probably near their limit, so I would not be surprised to see a drop in voltage across them. You should measure the voltage at VIN and VOUT on the regulator and at the input of your Raspberry Pi.
Adding a capacitor near the input to the RPi could help if the issue is quick drops in the voltage from surges in current. A capacitor will not help if the issue is the constant draw through the cables causing too large a drop. Either way though, trying a capacitor should not damage anything. Something around 100uF should be a good value to try.
-Claire