Yes, little Devon Rex sisters!
Yeah, 3 weeks is about the most I can do, no matter what I use. I find that it works a bit better with more litter in the box (a good 3-4 inches). Husband scoops in the morning, I scoop late afternoon, and then again before going to bed. They seriously poop like tigers, I don't know how they can create their own weight in waste every day lol
Every couple of days, I top it up with a bit more litter.
Do you still have chickens? Could the cats use their enclosure as a catio (and do their things outside sometimes)?