Here's what I'd do now that you have the sender out
1....Adjust the sender to about 23--25 ohms with your meter. Use the ohms "200" setting or whatever the "low" setting is on that meter. Tape, wire or otherwise "fix" the sender so it won't move.
2...Clip the sender with test clip leads right to the cluster ground, and right to the gauge sender stud. Turn on the power and see what it reads.
3...If it does not read 1/2 scale, measure voltage UNDER THIS CONDITION above from the gauge "power" stud to ground. You should see your 5V regulator voltage at that point.
I'm still thinking you have a poor ground or a bad connection in the sender wire, or the circuit path from the harness through the pins to the gauge connection, etc etc