Best Litter for kidney cats?

Hi everyone.

Oscar has had slightly elevated kidney values for just under a year. Not bad for a boy who'll be 19 in January. Cole is 16 and his pee biscuits have started growing a bit.

I use a litter made from ground corn husks which has been great. It clumps, but not quickly, and I'm going through more than I used to.

Does anyone have any suggestions for a type of litter to use, even if you don't have a brand name?

Much appreciated!

Sincerely,
Me and my Thinning Wallet!:D