Wigglyamp
The installations you describe were done by an idiot.
Now, I accept that most avionics shops are just wiremen, following diagrams with little functional understanding and close to zero signal-level understanding.
So, where does that leave us?
If you say that EASA exists to enable an idiot (who can squeeze a crimp tool) to carry out the installation, that is one line of debate we could explore.
If you say that EASA supplements a competent installer, that is IMHO a much harder line to argue.
The whole process dates back, I presume, decades to ICAO practices which mean little today. Just think about this: what does it mean to say a GNS430 is approved for a Cirrus SR22? A 430 could be installed in a C150 or in a modern King Air. Introducing the airframe or other equipment into the 430 approval is like certifying a steering wheel for a 1976 Viva, S/N 223367 to 223450 only, when actually it can (obviously) be used on any car with the right splined shaft etc. But no certification can cope with the huge variety of possible configurations with interconnected equipment - unless one assumes the installer is an idiot in which case every installation is a Major Mod.
The whole thing needs a drastic overhaul, but the widespread lack of competence in the avionics business is playing into EASA's hands.