Hey Skip. That is sage advice and I took it. Over the last day, I read all of the manuals for the parts I listed at the beginning of the thread. While I could be missing something, I honestly didn't see anything missing I was counting on even with the GNX375. Yes, I get that if I had 2x GPS navigators then I wouldn't be able to crossfill between the two, but I am only planning to have one and use the G3x touch GPS as a VFR only backup. The only missing feature that I might like to have is VNAV on the GNX357, but VNAV is available for VFR when using the G3X as the GPS source. Changing the GPS source is just 2 taps on the G3X touch flight plan screen.
Here is what I was able to confirm by reading the manuals + watching YouTube demonstrations of the functionality:
With a little fuss flight plan info can be entered via G3X, Foreflight on an iPad, or directly into the GNX 375 and synced to all 3. IFR waypoints must originate from Foreflight (to the GNX 375) or be input directly into the GNX 375. Looks like there is a work around for this in playing with the GPS source on the G3X, but not counting on it
GNX 375 transponder is controllable from G3X display
GFC500 will fly coupled LPV approaches including glideslope
GTR 20B allows inout of frequencies via G3X
GNC 255A allows input of frequencies via G3X
GNC 255A show ILS guidance + VOR course deviation on G3X (just like on G5)
GMA 345 can't be controlled by the G3X, but I want to keep it because I like having some buttons for dealing with audio quickly (such as one button clearance playback). I wouldn't loose that if g3x died completely.
GFC 500 couples to G3X for VFR VNAV, but not when using GNX 375 as GPS source
After thinking about it a bit, I actually like the 7" in front of the copilot. I fly with pilot friends relatively frequently and it would be helpful for them to be able to manage the flight plan, radios, TXP, etc.
What else am I missing / should I expect from spending this kind of money on a new panel? Also, if anyone has direct contradictory experience, please let me know. I am going only off what I could find in manuals / saw visually demonstrated on YouTube.