+1 Colum
I use Nova ME 3mm sinking that i get from Ian at Ideal (link Colum provided) but also still use flakes.
The other option is Nova FF 3mm floating but note that GF can have issues with air
(hence good idea to mix it up).
Fish growth is basically protein driven, but goldfish do not like too high protein.
And good idea to keep vegetation in their diet.
for encouraging fish to be a bit more bold try adding some duckweed or surface floating weed. If you get a few patches you will find them grazing it and it is easy to put some flakes in amongst it.
Will use a bit of nutrients if gets sunlight but not that much to be worth worrying about.
If your FT is dark then you may have to grow the duckweed in a separate sump/tank.
GF are fairly inactive in the colder months, and the fact they are OK at the bottom means that water etc is all fine (better than gasping at the top
). As it warms up they will move around and feed a lot more.