Fuel sender issue

A sender with 2 wires, signal and ground, wouldn't require the OEM chassis ground jumper on the fuel line. If your probing around had grounded the signal wire a good gauge would have went to max full position.
What we don't know... Ohms range of your aftermarket sender ( should be 80=E / 10=F ),
Tanks capacity and how much fuel is in it.
Keep in mind that most aftermarket senders are linear. They'll send approx' 45 at 1/2. The OEM thermal gauge looks for 23 to get 1/2. To get the OEM gauge anywhere near accurate throughout the range you will need meter match module.