With everything replaced including the pickup tube, it has to be the new pump. That is unless there is a fitting which is clogged.
When the pump was replaced, you didn't happen to replaced the pump rod which rides on the cam? Hope not
I have yet to see "and would like to see" what the shift foot looks like when its fully locked in forward gear.
Is it dead nuts straight forward or off to one side
Great pic of the foot showing when its in reverse, but no pic of when its in forward. The reverse looks like its one tooth off, as in going to far (why it needs to be pushed wih your finger)
Plug, drive vent plug
The plug on the top section of the drive which is removed when the drive is being filled
Put your fingers inside the gimble bearing and rotate it. See if it feels a smooth as silk with no roughness
I would say it was not, or something was damaged when putting together.
Disconnect the cable at the shift plate and see if the cable can be moved until the prop locks in the CCW direction. Then look at the shift foot and see if its facing straight forward
The pump doesn't suck much at all, and putting it in a tote even that high is not enough. On the hard it should always be with muffs
Since muffs do not work its either the muffs (cheap ones, something wrong with them) or there is a restriction between the drive and the pump