As my dad is on dialysis, he has gone through periods of having a very low potassium and phosphorus diet. For phosphorus, he took a binder tablet with his meals to help mitigate against some of the phosphorus getting into his blood- is there a doggie version maybe?
To reduce potassium, some food can be cooked twice to remove a lot of it, for example potatoes, we would boil for 5 minutes, rinse, and reboil in fresh water, same with lots of other fruit and veggies. Maybe you could make some homemade veggie treats for her, which have been leached first and then baked in the oven so that they have a bit of crunch? Obviously worth researching first, I'm just making a direct human comparison here.