It's one of the more interesting if unsexy engineering problems I guess. You mention small boats - composting toilets are making inroads there too.
I happen to live in the bush with a composter. As I'm also on rainwater, which was damn scarce earlier this summer, flushing toilets make me cringe.
However, I don't think that model fits the American mentality well. One more likely scenario would be the electric or gas powered combustion toilets: https://shop.ecojohn.com/products/tinyjohn-waterless-inciner... https://incinolet.com/
You see these in places where fresh water may be scarce or sewage treatment limited, e.g. a small live-aboard boat.
But it's really a question of what you want to optimize for.
As others have said though, a great easy addition to the current American toilet is adding a bidet.