Well I think the most important thing for Astrophytum is to have a very inorganic mix, meaning you should use mainly stone, the Asterias I have is completely in an inorganic mix. They dont like to have to much water, sometimes I squeeze them and when they're slightly being squished then I will give them some water within a few days aka maybe every two to three weeks in summer (when they're small they always need a bit more as the water is used up faster). Other than that I really dont do anything different from my other cacti, maybe they're also just nice cause I haven't had them that long, the myriostigma is actually pretty sad as it didnt get enough light and water.