I just rooted and flashed my phone with LG Marshmallow, because I'm heading out of the country for at least two months next week and there's no telling how long my network could take to finally approve 6.0. Some people might say this wasn't a smart move given that I'd never owned an Android phone before and I've only had this one for 5 days, but now who's laughing.
It was a hell of a process though, because so much of the advice I found was contradictory, I hit problems during the dark command-prompt hours which made me fear for the whole phone but I got there in the end.
Once you get the bootloader unlocked and TWRP 2.8.7.0 onto the phone, it's easy. TWRP will even prompt you if you haven't rooted the phone yet or didn't get it right, and all do is assent to its question and it does it for you. I got quite badly tied in knots over the various versions of ADB and Fastboot that people punt around in these threads, so I'm confused why people don't just install the official Android platform tools from the SDK section of the Android Developer site. Granted, you don't need everything that's in the Tools package but it has the advantage that it works reliably (which nobody else's copy did for me). Also, LGUP proved to be the simplest way by far to add a custom ROM (in my case, the genuine LG Marshmallow). It's a little bit puzzling that it wants to find a .tot file at first, but once you change that default to look for .kdz instead it's fine.
I made an effort to get the software that was particularly marked for G4 H815. I don't know how necessary that really was, since it seems like a lot of it is just general Android stuff, but better safe than bricked, I guess. Are you aiming for a custom ROM other than LG? I thought about it, but I decided that I'd want to keep it as LG since the camera is so special on this phone, and I wouldn't want to lose any functions.