It doesn't have to be 5 different materia every time. You can give him 5 of the same materia if you want, or some combination (3 of one materia, 2 of another as an example). Handing him 5 different materia is only needed if you're trying to narrow down RNG on what you get back.

I agree that it would be nice if there was an option to remember the last combination of materia submitted during that session so we could resubmit it with a single click, somewhat similar to how desynthesis works with stacking items. There could be a Resubmit button for those who like to play closer attention to what they're getting back, and a toggle to automatically resubmit.