One thing I have found I can't do without is a dedicated video recording app. FilmicPro or MoviePro makes a huge difference in complex situations. Locking the exposure and focus or adjusting the white balance, adjusting frame rate to avoid light flickering etc. It is easily worth the price tag of the apps.
One of the few apps I've ever payed for is FilmicPro, and I agree it makes a big difference when filming a concert. Although, with the iPhone 7 Plus it was more of a, "what would have looked like shit, looks sorta OK" when filming with FilmicPro. Hoping and anticipating that the 12 Pro will look way better.
4K video does seem to suck a lot of power and using the lightning port for a powerbank blocks it from being used for mic input. Do you expect the 12 to have enough juice for a set?
I think with a full battery, you can get pretty close to two hours maybe? Word of caution, in one of the cases where I was shutdown early on with my point-and-shoot, I pulled out my iPhone and shot probably an hour and a half of video. During the end of the encore, I could see that I only had a tiny bit of juice left. I should have bit the bullet and stopped shooting video, but I let it go. It ended up shutting off on its own and corrupting the video. I could play the video back on my phone, but pulling it off from the phone was a nightmare, and after I finally did get it, it was pretty much impossible to get any program to accept the file (it wouldn't open in Vegas Video, VLC, etc.).
what do you plan to use for microphone?
I always record audio separately with a high quality audio recording rig. So other than having it as a reference for syncing alternate audio, I'm not concerned about the camera's audio or its audio recording capability.