We adopted a very very fat cat last summer and she had piddle issues. I now have new flooring as a result (one of those "unfortunately-fortunately" situations I guess). But maybe some of the vet tips we got could help you?
The best being...give them room to go. Ours is a fat kitty (14 pounds of loving sweetness)..but the vet said even small cats have room issues. Most cats do not like hooded boxes. We never had a clue as our older cat, who we had since a kitten, always had hooded and no issues, ever. Plus the size of it. We got rubbermaid-type low sided boxes...big ones about 18 inches by 24 inches. We tried different litters, let them choose one and scoop daily as well, like you said.
Maybe your kitties, like our fatty-girl, needs more room...wide open space, to turn and turn and turn..then to dig and dig and dig some more? it was amazing how it worked for us. CLICK..and it was all better. Then I got new floors (ours was peeing on carpet, but there was older cat intimidation going on as well..all better now)
I have also decided figuring out the kitty-psyche is what it must be like for men trying to understand us women-folk! LOL
Good luck!