Amaranth + Mortar and Pestle should result in cornmeal instead of flour. Flour can already be made with 18 items excluding amaranth while cornmeal can be made with almost only corn. Cornmeal products are used a lot in Mexican cooking, but during Mexico's pre-Columbian period The Aztecs cultivated amaranth as the staple grain crop where it was used to make tamales and tortillas among other things [1]. Meanwhile, amaranth is not suited to leavened bread and cannot be used as a full replacement for wheat flour because it lacks the gluten [2].

[1] https://medium.com/@cholaconcello/foods-of-the-americas-amaranth-the-outlaw-grain-946957d9e51
[2] https://www.sciencedirect.com/topics/food-science/amaranth-flour