Well congratulations on a successful surgery - wow, that must be a life-changing event.
I'd guess the hazard of riding after a kidney removal is mostly risk of bleeding - if you haven't had any complications and the doc didn't object it seems a smooooth ride without too much exertion or too much jostling around should be OK.
After I had my appendix out (not trying to make comparisons with your surgery), I told the nurse that I was signed up to ride a century in a couple days; and I felt so good I might try to do it. She was shocked, the doctor thought it would be OK. But the nurse said if I happened to crash or make an emergency move, I might pop something. So I started riding again pretty soon, but it was a while before I stopped being super careful.