This is most likely correct, you can fix this yourself by removing the starter and cleaning the contacts (until all mating surfaces are bright shiny copper) or replacing the contacts.
Scroll down for the video, it's easy to do and cleaning them...
Check connections to the gauge, also check to see if the wires are chaffed anywhere and are shorted to ground (check for continuity to ground). I don't have access to the wiring for that circuit now, but it's likely one of the wires is designed...
You may try loosening the screws on the 3d printed bracket by about a half turn, the mirror still needs to be in their tight, so don't loosen them too much.
Have you replaced the distributor cap and rotor?
Fairly cheap, easy to do and not a waste of money or time to replace them after all these years.
Mine had an intermittent rough idle and replacing the cap and rotor made it run silky smooth again.
I agree with you that it may be related to battery voltage.
1. What is the voltage at the battery after it's sat for a few days?
2. What is the voltage at the door lock actuator after it's sat for a few days (when it's being actuated)?
3. What...