What are you missing? That it is not natural for a cat to go to the bathroom the same as a human. Toilets are designed for humans, litter boxes for cats. Your cat can easily fall into the toilet and when their paws are wet, they may not be able to get out. Then you will have to turn to a litter box as your kitty may never use the toilet again due to the fear.
As for the kitty litter, how about purchasing the flushable kind (I have never used it so I can't comment on the quality or recommend brands)? As for bringing it up to your unit, have a plastic container in your car that you can put the bag/box in so nobody sees it.
Cat maid to:
Jasper, male Ragdoll ?? (approx 10 yrs)
Sweet Pea, female silver tabby DSH (approx 8 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 7 yrs)
Puddles RIP (1996-2014
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey