How much baking powder & baking soda do I need to use for my cocoa cookies?

You don't strictly need to use any leavener in this type of cookie. You can certainly get a firm, nonchewy texture without it, although it is unlikely to be "crunchy" and not very airy - it is more sandy, melts-on-the-tongue thing. Of course, if you use crystal sugar (as opposed to powdered sugar) you will experience some crunch from the non-melted sugar crystals, which might be what you wanted.

If you want to try a leavener, a typical amount would be 1 teaspoon (around 5 g) of baking powder for 250 g of flour. Use that as a starting point for the recipe development - I don't think you have to reduce it to 4 g from the start, since 1) traditional recipes are not that accurate anyway, and 2) the cocoa powder also adds starch, which kinda "counts as flour" for the purpose of calculating leavening. There is no reason to use any baking soda. And of course, remove the ammonium bicarbonate - if you really want to experiment with a combination, do it after you have created a working simpler version of the recipe with just baking powder.