I have been following this thread with interest, as I too mod airports. One thing I have noticed (certainly true for previous builds of MSFS) is that to get ILS to auto-tune, one has to add ILS data to a re-built airport for auto-tune to work.
Without wanting to in any way disagree with the approach that @WombiiActual is taking in making an ILS work, a few months ago I did post a slightly different way that I use to successfully get an ILS to work for a modded airport.
The post is here: https://forums.flightsimulator.com/t/setting-up-ils-for-a-new-airport/375687/5
I am still confused in my own head as to the reliability (or otherwise) of the MagVar modelling in the sim and for the method I have described above, MagVar isn’t necessary from ‘real world’ data!
However, it may be that my method is flawed and that future iterations of MSFS may ‘break’ the method I have used to create an ILS.