Welcome to the LG world. Yes, android 6.0 was the latest LG released for most G4's - it was only the F500k (I think) that ever got nougat.
Factory reset sounds like your best bet. If you want more up to date android, custom ROMs are the way forward
I recently checked my G4's latest security patch (due to the announcement of the Krack Attack) and discovered that it's Android Security Patch level is 2016-07-01. I checked to see if there are any updates, and none are listed. Is this normal?