Turn Signals

With a US 7 pin connector there isn't an option for what you want.
I wish the US would go to separate yellow and red lights.

Hear, hear on that statement. With my old 2005 GM 2500 and old trailer we had amber rear lights to tap into not a problem. What do they do when they sell trucks to the UK for two types of mouldings? I did buy that unit where you break into the RV wiring to do what I wanted but I switched the hazarded lights it failed to work properly.

I don't believe the European/ISO trailer connectors have a brake wire. Much smaller trailers.
The Australian connectors do. They don't have a 12v charge wire but use that pin for the stop lights. I wonder how the GM truck wiring is for their units.......

GM Upfitter Guide 2015 page 193shows that this is controlled by the BCM.
Perhaps you could get it reflashed to an Australia RPO code ? GM RPO Codes
It looks like EFILive stops at 2014 model years for BCM reflash.
It looks like you are stuck with the 2-3 wire conversion box that you have. I mean, how often do you have the hazards on anyway ? 😁