revisit the fuel gauge and sending unit issue

So here's the deal, theres only a few things that can be wrong in your case. Let me list them out.
1. Sending unit ground. You said you have 2 ground points on the sending unit, that should be enough. I've seen several Mopar sending units work without the ground strap present. I doubt that's the issue.
2. Wire integrity. You said you ran a dedicated wire and saw no difference vs the stock wire. This also probably isn't the issue.
3. Gauge. You said it's a new aftermarket gauge, so it most likely works. Is the gauge linear or non-linear? What is the gauge calibrated for?
4. Sending unit. You said you bought several sending units with no difference. Did you ohm each of these outside of the car with the same results?
5. Sending unit position. I've seen several aftermarket sending units where the pickup tube wasn't in the right place, it was either too high (most common) or too low (you can tell that easily because it won't seal correctly with the gasket/locking ring). In addition to this issue I've seen improper movement of the float arm, such as the arm won't go near the bottom of the tank, so your tank is over half full but the gauge shows almost empty. This can be fixed by bending the arm. You would need to have the tank out of the car and look through the filler neck hole to check this. Me personally i've never gotten lucky with aftermarket sending units, each one I've installed I have had to do it with the tank out to ensure that the pickup tube reaches the bottom of the tank and that the float arm is going through proper range of motion.

Have you considered just getting an OEM sending unit and being done with it?