Unless you already have the G5, going with a D10A is the shortest path to getting your system installed. The Dynon STC calls out a D10A as the backup instrument. If you don't install the system as called out in the STC with all the required components, the STC is invalid.
If for some reason you just really like the G5 and can't live without it, you could install the G5 using Garmin's STC in addition to the D10A.
The other course might be to try to get a field approval to install the G5 as a substitute to the D10A. You might be able to make a case that the G5 is functionally equivalent. Since the D10A is intentionally kept completely separate from the rest of the SkyView system, the substitute wouldn't impact the operation of the remainder of the system installed under the STC. The FAA might buy that argument. In my experience, the FAA field offices are more like franchises than company stores. There is FAA "Guidance" and then there is local district manager policy. The exact procedure may vary, but you might be able to get a field approval to substitute the G5 for the D10A and you might not.
The best bet is, before you do anything else, you or your IA should set up an appointment with the local FAA FSDO and/or ACO to discuss the substitution. They are much more receptive to having someone ask for their guidance rather than to a finished or committed project presented to them for approval. They tend to take the latter as dairing them not to approve your project. Depending on how they feel about it, they will be able to tell you what your options are and if or how you can proceed from there..