Help needed with litterbox training?

I'm an experienced cat owner who just adopted a kitten rescued from a devastating animal hoarder situation. There were dozens and dozens of cats in the home and ZERO litter boxes. Tori was pulled from the home on 3/25. Urine and feces were caked so hard to her paws and legs it took considerable effort by shelter volunteers just to get her clean.

I brought her home last week and her paws are still extremely tender. Kitty litter hurts her feet so she won't use the box. The good news is that she's doing her business in the same place - the bathtub, a super easy place to clean. Also, it's a clear indication she wants a spot to call her own.

I'm thinking about getting some extra fine sand to try transitioning her to the regular litter over time. But I'd like other cat people's ideas first in case there's something better.

Any thought? Suggestions?

thx