LED's flicker, Idles rough, now nothing at all

Battery ground cable - where it connects to the block, or frame, not at battery end. I've seen these corrode inside, under the insulation at the battery end, also. As a diagnostic, you might connect a jumper cable from the negative battery post to the engine block. Do not crank. But now see if dome light, headlights, and so on work. Then disconnect jumper cable. If they go back out, you know the problem is the battery grounding cables. If (with the jumper cable) nothing still works, the problem is elsewhere (including possibly the POSITIVE cable. I've also seen what you describe when the ignition switch fails. Some circuits do not get energized. See what others have to say. My experiences (with the switch) are on vehicles 5-8 years older than your Tahoe.