i'm not an avid shiny hunter so i don't have much say in this, but couldn't it have been more fun to just add more pokemon to the horde pool? like if you want larvitar to be 20% horde add onix, graveler, rhyhorn and sneasel hordes or something. this way the rarer shinies are still rare but atleast you dont have to go through 4 onix and instead you go through onix. graveler, rhyhorn, sneasel for some variety. and you just make the shinies you want to be more common avaliable in more areas.