Maybe I missed it but I don’t see where you said you checked the grounds (beyond cleaning connections last fall). It’s easy to overlook the simple things sometimes.
The daily High on Friday this week will be 5 below at my home in south central Wisconsin. I’m in Florida for a couple weeks right now and it’s 73. Not looking forward to returning home on Sunday!
Check the starter solenoid and main ground for clean and solid connection. If they are good, pull battery and take to auto parts store for load test. Beyond that, possibly a bad connection in key switch or starter button