Maybe do a voltage drop test from that post to the other end of that cable. A bad connection could definitely cause your symptoms.
Voltage is not all you need. You gota have amperage too. I have seen alternators produce 14.0 volts but only 2 or 3 amps. THAT will not charge a battery ! You need to check them both. Also test a battery with the same load as your starter draws . If your starter draws 200 amps while cranking then that's the load you put on the battery. The ONLY true check is to make it do what's its supposed to do not some electronic bull crop meter.
East Penn/Deka batteries-NAPA or maybe Parts Plus?
I saw Lowes carry Deka otherwise autoparts store?
What wrong with using a vice grip?
And just leave a vice grip on the battery lug permanently?
That was said very "tongue in cheek" because I know you are fussier than that. I do a lot of fixing for farmers and I have seen many lets say interesting repairs.