Adding the AP74 will make your AP operation a bit easier with dedicated AP buttons, but there is some overlap with the buttons and knob on the HS34.
The AP74 does not control electric trim, so you'll have to add buttons and a relay deck for that and it won't interact with the AP. You may be thinking of the SV-AP-PANEL for our SkyView system, which can replace the relay deck and provide AP Auto-Trim (but only works in a SkyView system, not a FlightDEK-D180).
I have an RV-8, HS-180 with HS-34. Built with manual trim. I am thinking about adding the HS-74 autopilot. Is there any reason to or not to add electric trim? John
