Hello Mark SF, The Raymarine system is easy to assemble although for what the cabling and cable connectors are I feel the components are a bit expensive.
From what I have read the NMEA standard only covers the number of wire and wire colour in the cable.
The method of locking the cables to the instrument, T pieces and connectors within a system is not to a standard but left to each company to devise their own method.
As a result there is a variety of ways to connect such as male cable / female instrument and vice versa, some use threads to join, some use a lugged ring that locks into a corresponding groove on the male cable end etc.
I am sure you get the idea. So it makes it difficult to join an instrument which has say a male thread into a system that uses cables with a male lock ring end.
As you said , "get an adapter" and that's where the problem lies. Who makes an adaptor for say a raymarine cable (male lock ring end) to join to a male thread Garmin
instrument. No one that I can find yet.
The only solution I can see is to get a raymarine cable and a garmin
cable and with a bit of cutting and joining join both cables together, possible thanks to the wiring