I think I understand the problem of the free turbine and its solution in the context of MSFS.
It looks to me that MSFS is considering a turboprop like a turbojet with a prop attached to the end.
I’m thinking it could be possible to get a “fake” free turbine by considering the N1 as N2 and N2 as the free turbine. Then it would be possible to use the engine tables to tweak the relationship between N2 and the free turbine, set the gear ratio to 1 thus having prop RPM depending on N2.