I also use FF premium classic and also have zero problems germinating in them. I repot twice in veg, so no need for nutes. On the second repotting I mix some FF & Elemental blend into the bottom of my final pot at a ratio of 3 parts FF to 1 Part Elemental blend and let the plants recover for a week, then i flip them. And again, like you, I top dress in week 4 of flower with the same ratio of FF to Elemental blend. Seems to be working for me.
I also add lots of perlite to the FF as I find it tends to compact after a few weeks.
I even use the FF to make a pseudo compost tea once a week.
Would like to try the Orgasoil next, though.