I have a similar setup (D-100/D-120, HS34, GPSMAP 796, SL-30, TruTrak Gemini) and have been looking into adding a GPS navigator (Garmin 375). What I have found out so far may be of use to you.
1. According to Dynon, the HS34 does not "pass through" the arinc 429 so the 429 for the TruTRak has to be wired directly to the A/P (cannot wire the 429 to the Dynon Chan 1 RX and use the Chan 1TX to output to the A/P).
2. On the Gemini the autopilot defaults to the 429 input and if that input goes away it reverts to the RS 232 guidance if available so if both are connected the 796 acts as a backup, this may be the same for your 385, it is referenced in the manual on the Gemini.
3. I doubt the servos are compatible, every time I have checked on upgrades it seems the servos have to be replaced, a lot of the modern servos seem to be CAN BUS based. My D100 was installed before Dynon had their A/P software done otherwise I would probably have put in their servos for a cleaner A/P interface.
4. If all the inputs are connected to the HS34 then I believe you can select the sources, on my current setup I can select between the SL 30 VHF Nav and the 796 GPS Nav. I think the selection steps through the available sources that are connected.
Hope this helps.