Chiming in on this thread since I'm having the same issue on my 2007 Escalade. Normally the threads I find from google are a decade old so it's nice to see a current one.
I had exactly the same issues; no crank no start with the missing prndl indicator, various other circuits not working like exterior and interior lights, radio, tailgate, etc. Never had low battery voltage with those symptoms.
Like you, disconnecting the battery fixed everything but sometimes it would take a dozen or more tries to "reset" it. I could never figure out the cause.
At first I suspected a ground issue but I had gone through all my grounds, plus the fact that since I have a high powered audio system (5000W RMS+) all my grounds are absolutely solid. 4/0 OFC under the hood and dual 1/0 OFC to my rear lithium battery. Amplifiers grounded with the same. I had removed the factory battery ground since I no longer had an underhood battery. In addition to the factory grounds, I had added block to frame, block to body, and body to frame under the hood and had battery to frame and battery to body in the rear.
Then I found some info on the junction block (underhood fuse block) having issues with corrosion or bad board traces or something. My coworker had the same issue with his Tahoe. I removed it, checked for any loose terminals, cleaned it up and reseated it. That managed to fix my issue again for a bit, but I still had to disconnect the battery a few times to fix it again.
Last week, I had an occurrence of the issue that likely completely killed several cells of my lithium bank (dropped me all the way down to less than 6V on a 13.8V bank), or several of my cells died all at once and caused the issue too occur(?). Replaced it with an AGM and no amount of fiddling with the ground cable or junction block fixed it. So I replaced both the junction block and the base since some of the tangs that hold the harness connectors were broken. Still have the same issue.
Today, I went out with my tech 2 + CANdi module and checked what I could (battery was at 11.8V from messing with everything so much so I'm waiting for it to charge in a few hours). Turns out I had connection with the TCM, liftgate, radio, ecm, and all the stuff I thought wasn't working for whatever reason. The only thing I couldn't connect with was the BCM. BCM controls a bunch of the stuff that isn't working and also I think the transfer case module and some other things related to actually starting the truck. I still have to find the wiring diagrams. I'll be going back out in a few hours to investigate.
So your issue still may be grounds, so definitely disconnect everything, sand down to shiny metal, apply dielectric grease for protection, and also check the resistance of the wire to see if there's any internal corrosion. I'm not an electrician but my grounds were in the 0.1 to 2 ohm range, including through the frame from back to front. I think I read somewhere that less than 5 ohms is acceptable, but someone should correct me if I'm wrong.
Or your issue could still be BCM, since GM was very bad with soldering (just ask anyone with a 98-00 Corvette that had ABS issues) and that would make sense why you have the issue more often in direct sun rather than in the garage.
But definitely check all your grounds first and I hope you have some good news when you reply. I hope I have some too.