All the connections look OK and the output jack is correctly wired.

What solder did you use? The pre-wired stuff will be using lead-free solder. I still use 60/40 lead solder at home as it's easier to use and melts at a lower temperature, so is less damaging to electronic components. But it doesn't mix with lead-free solder, so if you are using leaded solder, then I'd make the back-of-pot connections on a different part of the pot to be sure of a good electrical connection.

But if your ground path resistance measurements are OK, then it may be down to a damaged volume pot (though I've not experienced this particular issue before). I'd be tempted to get another couple of mini pots (250k log/audio short split shaft - 18 spline) and re-do the controls. I'd also modify the pickup leads so that there was a short length of braided wire that goes to the rear of the volume pot and a much longer length of the insulated central core signal wire that runs to the selector switch. It saves having to use masking tape to stop any inadvertent signal shorting.