Wet litter sure is heavy, isn't it? I live out in the "boonies" and there's no trash pick-up. We have to take our trash to a "transfer station" where we throw it down into a trailer that takes it to a dump. One of my neighbors has nine cats and spends probably $15/month at the station just in cat litter weight. I, on the other hand, put mine around trees and bushes (no kids here
When I used regular litter, I took it into wooded areas (I have 12 acres). Then I switched to scoopable and I hated it, because it never seemed to dissolve and turned into cement. Now that I use chicken feed, I fertilize the shrubbery with it. One rain and it's gone!
There are "flushable" litters. That might work for you as you have just one "cat's worth" of litter. I'm not sure I trust the 'flushability', though. I have a septic system instead of public sewers, and I'd be afraid of messing it up. I'd bet any money that the chicken feed could be flushed, as it is basically crushed corn, but with six cats, that's a lot of stuff to flush.
Since you have a garden, my suggestion would be (1) switch to a non-clay litter (corn, paper, or pine-based......but corn or paper would be best) and (2) compost the litter along with your other "compost-ables" for the garden.