I wonder what voltage drop you would get if you measured it right at the battery terminals while cranking the starter. Is the battery charged up real well? A LITTLE voltage drop would be expected but I don’t know how much drop is acceptable. If you took voltage measurements at the battery, then at the white wire connection at the starter, then at the thermal timer, you might be able to isolate where you’re losing it. Could be a nearly severed wire or a bad crimp job or ???

I don’t know about the “all or nothing” theory at the thermal timer. As a test you could bypass the thermal timer with a jumper straight from the starter to the CSV and then ground the other CSV contact which would essentially take the thermal timer out of the equation and then see if the CSV opens when commanded (starter cranking).