I'm a pellet girl
Flakes are much harder for goldfish to eat. They're good when they're very small, but once they can fit a mini pellet in their mouth, that's a better option. I've been feeding all my goldfish pellets from the beginning (Of course combined with vegetables and live foods), and when I tried to give them some flakes that I have for my betta, they seemed to really struggle trying to catch and eat them.
If you're worried about the pellets sinking, you should rather get floating pellets and soak them for a few minutes before feeding
I have a couple of really good brands that you may want to try out!!
JBL Novo Pearl, floating, meadium-sized pellets that soften quickly after only a few minutes of soaking
Hikari Staple Goldfish, floating, mini pellets that don't ever sink, but use to soften up in water. If you want to soak them, you'll have to do that for at least an hour or something D8 (Tried and failed...) However, these pellets are really dense, so it won't be a problem for the goldfish to eat them. Hikari Goldfish Gold is another type of the same pellet, only with color enhancing qualities.
Sera Goldy Royal, slowly sinking small pellet, really nice for the goldfish to eat since it contains extra nutrients so they'll grow better.
Generally about the sinking food, feed it slow, so your goldfish will see it and be able to catch most of it before it hits the substrate
But every goldfish loves digging in the gravel anyway, so they shouldn't have any problems finding the leftovers
Try not to give them too much either, that would pollute the water even more and more food would sink to the bottom.
Hope this helps!