I used silica years ago and noted that it never hid the smell of poop. If you want something that masks it and don’t want to use clay, quality wood pellet litter can be good. Personally we use low tracking clay because it is what has worked best. I would mention it to the vet that the poop smells more potent than past cats. It could be health related as someone mentioned. Litter box choice? I don’t think matters for smell much. I have had several different designs over the years (no mechanical ones) and they all function about the same for smells.