Couple of things. It seems like you have a lot of resistance somewhere. Sounds like your terminal/cable.
You mentioned earlier about buzzing from the passenger foot well area. I was also going to mention a smart junction box, but didn't want to jump to conclusions.
My SJB went slowly. Expy would not start. I also had buzzing there. I popped the hood and thought it was a terminal - felt a little loose but corroded - could not tighten. So I gave it a serious whack with the butt of a ratchet wrench and I was able to start the truck.
I have a spare 'iffy' SJB you are welcome to try out. Don't laugh, but I pulled all the fuses put them in a bag and taped it to the box. It's in the cargo area. I'm sure you are capable of reinstalling all the fuses. I keep them as back up.
Are you still in Spring? I'm in Houston. Send me a PM to chat or cell to call on Tuesday if you want to borrow it.
PS I went through some repairs several months ago involving similar issue - but my truck would start and stall repeatedly on the road. That involved both battery cables, starter cable dealer built harness/fusible link, Ford DPFE plus other crap. I tried an Amazon DPFE but dealer figured out it was faulty. Paid $$$ but truck is running great now. I also tried ignition module (not switch) and it probably had nothing to do with the issue.