My fuel gauge acted similarly, I was about to give up too but in one last try I took some sandpaper to all the grounds at both sides of each ground (back of gauge & where that grounds to cab) & (at sending unit & at cab)
It went from empty with a wiggle at start to floating somewhere in the middle (would be reading properly if my sending unit where functioning properly) its almost always grounds, even a clean looking ground may not be making proper contact
when you ground out the sending unit with the ignition in the on position does your gauge move?
not sure if adding an aftermarket fuel gauge will solve your problem. If your gauge moves when you ground out the sending unit it's still good and there is no need for a new gauge. It's likely the wiring.
