ahh. yeah the gm software is a mess. I had to watch a bunch of YouTube on it. it fails often and then all of a sudden works for no apparent reason. but you definitely gotta put a charger on the 12v battery while doing just about anything to these trucks that has the key on engine off. they kill a 12v really quickly with the key on. I did it myself even knowing better while doing brakes. when I put my amp clamp meter on it to test. it pulled something like 20 amps form the 12v with the key one click forward, not even with the dash lite up. gm calls for a 12v voltage maintainer, because a changer isn't always clean DC. they can have some ac ripple. a battery doesn't care while being charged but the vehicle electronics can.
I used a charger anyways, as it's all I had handy. but just info as to why they call out a maintainer. the update can fail for a many reasons, the 12v battery being low is definitely one of them. the hybrid battery maybe, but without a load on the nickel metal hydride cells, they always look OK. their voltage crashes under load if not changed up but can look good at rest even when dead or damaged. so I doubt that's failing your update, but it can definitely be why it won't crank.
I missed that you said you had a tech 2 and got it to work for a flash. that's good work. the tech 2 will allow you to get into jump assist mode. where you put a battery charger on the 12v and it will use the inverter to charge up the hybrid battery. it is very slow, so may take hours. 12v at 20amps turns into 330v at some milliamp charge rate at the hybrid battery. but it does seem to work, but can be finicky to active the mode. there's good direction pop up if you search for hybrid tahoe jump assist
definitely keep us posted
