Using Sourdough Discard - you can use sourdough discard for these sandwich rolls, just add a pinch of yeast (or a teaspoon if you need them in a hurry) to help the discard along.
Flour - I recommend using bread flour for these rolls as you want them soft, but still with a chewy texture. You can add some whole wheat in place of the bread flour but I wouldn't replace more than 200g.
Bulk Fermentation - you only want the dough to rise a bit during bulk fermentation, the rest of the rise will happen after shaping. So don't allow your dough to double, you just want it around 50%, no more.