weird ignition issue after installing tach

You need to start at "ground zero."

Disconnect the tach wire from the ignition, and I assume you hooked it to coil negative?

Or does this "ready to run" thing have a separate tach lead?

What this SOUNDS like is that when you rassled around in there hooking up the tach, you bumped loose the BROWN wire coming from the key, which used to go to the ballast resistor on the coil side.

When you twist the key to "start," the dark blue ign wire goes cold (IGN1, dark blue)

When you twist the key to "start", ign voltage is / was supplied by the brown (ign2) coming from the ign switch, through the bulkhead, to the + side of the coil.

Normally when you remove the ballast resistor, you must tie the dark blue/ brown together.