Poor kitty. Reminds me of my old cat. She made it to 22 years old. She could make it to the litter box, but preferred to go around the litter box. She looked ver much the same shape as yours. Didnt groom herself anymore, so we had to help her out.
She pretty much just slept on the bed and we keep a litter box nearby in the same room, so she didn't need to go far. She had joint pain, so I didn't want to make her go all the way to the younger cats litterbox.