Definitely lots of opinions, but you need to decide what is right for your flying. I would suggest the G5 too as a backup in lieu of a second 10" screen for the reasons already mentioned. Did anyone say it can drive the autopilot too.

Battery: You need at least one other battery other than the big main one. The #1 reason is to keep the G3X system/screen alive during starting; the screen will go blank and reboot if the voltage drops below ~10 volts. I originally had a second 5AH lead acid battery for this purpose (4.5 lbs with wiring). Last year I upgraded to a home made Lithium battery setup with a charger/balancer circuit board (the whole thing is under 1 lb.). This battery also powers my electronic ignition during the start when the main battery voltage goes low.

The other possible need for a second battery is to keep essential systems (G3X screen/sensors, ignition, lights, COM?, transponder?, trim?, etc.) alive if the main power source quits or dies. Again, you should read Bob Knuckles-Aero Electric information. You get to decide what is essential for your plane and type of flying. I designed my system (a single 10" screen G3X) so that I can keep going on the main battery for 2.5 - 3 hours if the alternator dies. I have a backup feed directly from the battery to the essential bus through an "emergency" switch. My main battery is an Odyssey 680 (~16 AH).

My independent backup is an iPad using the Garmin Pilot app. This gives me everything I need to get safely on the ground day/night VFR. The only thing I wouldn't have is engine instruments, but I feel the only critical one after starting would be oil pressure, and the chance of loosing that is about zero.