There’s nothing to stop you making your own nitro candy. It’s just dye in a carrier (clear lacquer) so you could buy HOK kandy concentrate (not the pre-mixed kandy)

The only downside I can think for using nitro with metal flake is you’ll need a fair few more clear sessions to bury it, but since the clear isn’t costing you it’s not really a big deal.

Check out tropical glitz metal flake, they claim to be the thinnest in the world. Paint huffer also make nice flakes.