Use instead of annatto

Add cumin, chili powder and garlic powder to your salt and pepper. You might like a bit of curry powder or turmeric too, but I don't know if you'd call that a Mexican flavor.

I don't think it is close to either cous-cous or rice. It is less starchy than both. Also, it has a unique mouthfeel when you chew on it, because the tiny grains (1 mm or less) pop under your teeth.

I would try getting the real thing in a whole food store or an organic store, or also online. If you can't find it, in these stores, but can get other exotic grains, you can try quinoa or millet.

If you really can't get anything uncommon, I would use durum semolina. Wash it before cooking and cook it with fat to reduce the starch. Also, I would maybe cook it in more water than needed and remove excess water after it has been cooked and cooled, so your semolina grains don't form a creamy pudding.

Maybe tapioca pearls of the extra-small variety can work too, I am not sure because I haven't tried using them and don't know what their final texture is.

You will notice that, when suggesting replacements, I am most worried about texture. The reason is that the flavor of amaranth is very faint and bland. It is best used as a base for other, strong flavors. So you can use anything with the right texture, the flavor in the dish should come from spices and other components.