Would it be possible to make DIY aquarium-safe paint?
I don't think it can be done, and even if you could it wouldn't be cost efficient at all. I'd suggest just investigating aquarium safe paint online and getting whatever is available to you locally
If it's sealed with polyurethane it's safe. 7+ coats, build coats with gloss then last coat at the sheen you want. You can also seal the paint behind clear epoxy, or use a dye-able epoxy.