Robert, I’m glad you are having some success. Based on my own experiences, here is something else for you  to consider.When I acquired my own car, I checked and cleaned all the various connections on the convoluted battery cables.  Went over everything, or so I thought. About a month later, I went to start the car, turned the key to the “start” position, and- nothing. Silence.  On strike!  I tried a few times- not a click. Finally, after maybe six attempts the starter engaged, and the car came to life.

I suspected the starter/ignition switch, and removed it from the car, and opened up the electrical part.The contacts had a coating of slight corrosion , and just needed  to be cleaned and lubricated. Not wishing to be stranded  somewhere, I wired a generic pushbutton switch into the starter circuit, and mounted it under the dashboard, within easy reach. Now I have a back-up. I find myself using the push button to start the car now.

I suspect a “specialist” will advise replacement of parts rather than a repair, but that is for you to decide. Corrosion inside the starter/ignition switch is fairly common, I have discovered.