Kitty litter is made from clay, there’s hardly a significant environmental impact from the making of it. I also don’t really think there’s that much of an impact from disposing of it into trash cans, but if you’re really concerned about that, you could probably just dump used kitty litter in a corner of your yard. Stick a flat stone or some dried leaves over it to stop any smell, and voila.
I’m glad you plan to keep your future kitty indoors. Much better for all involved. A tip for cat toys: you can buy intact quail and duck wings from places online that sell them so you can use the feathers in flyfishing. The wings are a byproduct of hunting, the birds aren’t hunted specifically for the wings, and the wings are dried to preserve them. Get one or two, and your cat will go /nuts/ attacking them, like they’re real birds. Durable, safe for kitty, and great enrichment. If you know anybody who hunts birds, ask them to save you a few wings, and fold the wings up like how they’d be folded on a bird’s side. Stick the wing in a garage, sprinkle a big handful of salt on it, and leave it there for a month or so, it’ll dehydrate and won’t rot.