Not sure if this helps, but you solder everything to the PCB except components in sockets (you do solder the sockets). And if you are happy with the given component you can even solder it into the socket for longevity.
I recently bought lead free solder and I noticed a significant difference. Leaded is much easier to use. I use a Pace set at 700F and I never had issues. The unleaded stuff sometimes looks like it’s not flowing but then the resulting joint looks fine so I don’t know what to make of that!
Mmm if you think the neck is straight maybe it’s the other hypothesis: the strings are choking out? Are the lower frets quieter when the guitar is unplugged?
Thinking about it, the fact that you get buzzing up until the 10th fret may indicate that the strings are choking, which may make them a lot quieter. Do you find those frets quieter than higher frets when the guitar is not plugged in?
I was just guessing based on the symptoms, I may be totally off. But I can’t think of how a pickup could cause what you are experiencing. Do you know how to measure neck relief?
Edit: you could also measure the distance of the strings from the pickups at different frets to see if that’s...
Yeah it sounds like you have too much relief in the neck and probably high action which, combined, would cause the distance of the strings from the pickups to reduce as you go up the neck, increasing the output.
Nice clean build! Would love to see one of your awesome demos for this! BTW, are you not worried that the leads to the power jack and the ground pads can crack over time?