Apparently I simply need to add system.prop to root of zip (see didgeridoohans post above)You literally just have to add what you want to the /common/system.prop file and set the tag in install.sh for propfile=true. Re-read the documentation, a simple file change is easy.
I forgot to add, I'm using mnt extended template and install.sh is a blank document in /common
From what I have gathered with my convo earlier, resetprop (presumably part of magisk) will load the props from system.prop on the root of the module zip.
As this makes sense and to avoid confusion I will try this process and return to yours if no success
Furthermore a couple of places are saying not to bother using install.sh due to resetprop