my advice would be to go and do the microlight instructor course
The Microlight Instructor Course was designed around the weightshift aircraft and that accounts for most of the differences. When 3-axis became more common, the Microlight World embraced them and in the main weightshift instructors moved over to teaching on them without any form of instruction on how to do it. For that reason I would certainly advise doing the Microlight Instructor Course if you want to teach weightshift, but I suspect that as a FI already, you already have a better knowledge of what is required than many Microlight Instructors.
Microlight operation and training differs from State to State and has nothing to do with EASA so you need to look at the requirements in the State where you wish to do it.