Daisy chain of grounds a bad idea. Grounds should star out from single ground, so many terminal are not in series. Measure the ground at tach, back to battery negative, with circuit powered. A good voltage there should be near zero.
That's two confirms that I may have a ground issue. My other few gauges seem to work fine but the tach may be looking for a better ground as you stated. Time for some better quality wiring. I learned a lot with this build for sure. Wiring was never my strong suit.....
