[cheshired5]

I'll start with the absolute basics.
Clean all fuel unions with brake cleaner so they are bone dry and look for leaks.
Can you hear the in tank pump running with ignition on?
When was the fuel filter last replaced and with what brand?

HP pump appears to working fine so I'm guessing the issue is around fuel delivery/regulation.

There's either not enough fuel available due to flow/restriction somewhere or fuel is available and not being regulated correctly at the HP pump.

Given the work on the unions didn't go smoothly, it adds an extra level of possibilities.