Your best bet may be to put these in a breathable container along with some compost and see what insect they turn into.

It shouldn't take long, weeks at the most, maybe even days.

You are likely regularly seeing the adult insects around your compost already, unless you are very good at killing all the larvae.

They look sort of like a Cranefly larvae, I think. Why are you certain that they aren't a species of BSF? What's the approximate size?