Could I use a levain and what would the ratios be?

Better, you can do that but it will alter the times also. I use a 50-50 flour and water by weight levain most of the time so I can adjust the recipe accordingly with easy math and stay on the same percentages. For instance, 200 g of levain means you take 100 g of water and 100 g flour out of the recipe, or if you want a big loaf or two larger small loaves, add some flour to get back up to the same percentage. I use the levain when it is fresh (still rising but nearly double). Nothing wrong with adding a little commercial yeast to speed things along but you’ll end up with less of the sourdough complexity if you take that shortcut. It is still incredibly good bread.