I have done two homes and several other heated buildings, in Fairbanks Alaska. They've been done a lot longer overseas in similar environments.
I've always used NFS fill down to frost depth, in conjunction with the foam per IRC.
Consider frost heave only happens when ALL three occur: Freezing temps, Frost susceptible soils, and moisture to freeze. FPSFs are designed to eliminate the freezing. Proper drainage and fill material above the foam can minimize the moisture, and if you do NFS soil as well then you could have any two conditions become compromised before you have an issue.
I've made spreadsheets comparing the costs given local fill price, and dimensions of the building. The FPSF always comes out a little ahead or more. It's worth doing your own local cost benefit analysis.