A le Ron there is no plan that I am aware of to make ADS-B mandatory - it is likely that it will make life easy but it will not be mandatory in class g. The cost of doing ADS-B out if the none certified changes are approved is very low unless you have a Garmin transponder. The cost of doing ADS-B out on my MCR which has an MGL EFIS and a Trig mode s transponder was the cost an rs232 cable.
Which Garmin did you get stitched up with? Some are upgradable.
"Surely the IR holder will very rapidly bump into problems" - like it - no pun intended?
Rod1