I have not made crackers with rye discard. I make rye bread frequently but use my white starter.
BUT, your question gave me some ideas. I think I will try some rye flour and a caraway-fennel seed mix in place of the herbs.
My advice would be to make the recipe a time or 2 with white discard if you have it, just so you get used to dough consistency, baking time under your conditions.
Now … starter. This recipe is for 100% hydration so starter of 248 grams is 124 grams flour and 124 grams water. You can use less actual starter and sub additional flour and water. Example, if you used 124 grams starter then you would add 62 grams flour and 62 grams water to the other ingredients.
All that to say, that if you only have rye starter, you might try … for a first recipe try to use 124 grams rye starter, and add the 62 grams flour as white flour, i.e. not quite so much rye to start so you could see how it behaved. If all went well, increase the rye starter and/or add more rye to the rest of the flour mix.