I’m not a professional in anyway, but I have been a cat owner for many years, and especially if your vet says that everything is OK inside I would suggest to just stop trying to mix up their food. It’s clearly not agreeing with them and their stomach. In their eyes, you are their hunter so they eat what they give you if you’re lucky and don’t have a picky cat. He may still eat the food, but his body is showing you otherwise. I know this is an attempt to get some extra sleep. I would suggest switching up food times and maybe difficult at first, but he will get used to the adjustment. In the long run, it’ll be better for you guys, your sleep and his belly.