Neil,
I have fewer avionics but do have toggles along the bottom of my panel for my batt, alt, EI and mag, avionics master, lights and boost pump; and agree with the other responders' comments above. I used strategic spacing a bit too.
I put all the avionics items in the photo below on the avionics bus, except for the annunciator panel, and organized my switches in groups by function. If your G3X is also the engine monitor, if its allowed, you may want one on the switched batt bus, so its on during start (I don't know enough about the G3X to make specific recommendations on that. I have a VM 1000 out of the photo on the batt bus for engine info).
I also put the switches in somewhat of a checklist, or operational, flow pattern. Samples of my flows follow, FWIW (just one technique):
Before Start is: Batt, Alt on, L-R with left hand, then Mixture, Prop, Throttle, Boost Pump R-L with right hand.
Start is: Mag, EI on, then Start button, L-R with left hand, then Avionics Master on after start.
Before Takeoff is Lights, Boost Pump, Throttle, L-R.
Shutdown runs the switches R-L to turn off (avionics before pulling mixture).
I just played with it as I did the panel, and spent some time with my checklist to set it up that way (did the same thing for a corporate Citation and King Air I flew a while back, so it was an old habit). I've found these switch locations (low on the panel) to be very comfortable, though its a personal choice, and many options are good. This one works for me.
Location of your pitot heat switch could go low as well, though you might consider an indicator light for it up higher in your field of view.
I was a bit of a goober, and made the switch covers the same color as the annunciator lites for the same function.
Have fun putting yours together!
Cheers,
Bob