9Aplus
Both the G5 and GTN750 are very capable bits of kit but it will take a lot of thinking about.
The GTN750 uses a lot of radio rack space and may have implications to your second comms radio depending on the route you go down with the audio control system.
I had considered the GTN750 and using the Aspen for AI / HSI display but decided to go with 2x GTN650 and G500 with the audio controlled by a PS engineering audio panel. Both the DME & ATC transponder will be remotely controlled.
Also the Garmin kit is compatible with a lot of comparitvly cheap traffic kit that will give you warnings of ADS-B, Mode S & FLARM equipped Aircraft.
This decision was largely dictated by panel space and equipment redundancy issues as well as the large (& costly ) display on the GTN750 Is mirrored on the G500.
I suspect that the radio rack space will not be as bigger issue on a C172 as it was on my Aircraft.
If you can find an avionics engineer who also flys with this sort of kit take his advice because people who understand both the flying and technical aspects of an equipment of like this are few and far between.