I would steer clear of the big box store plywood. Often it is imported, has thicker and fewer plys, and made of species with less density than is ideal for woodworking. I have disappointed myself a few times by doing this for "shop" items and have learned that spending a few dollars more results in more joy during the process of making as well as more joy in the results. Prices continue to climb but NCWW lists multiple sources for true Baltic Birch or domestic hardwood plywood. Even painted finish carpentry benefits from quality materials.
I generally don't design plywood shelving >36" wide because they will sag. The exception is when the shelves incorporate a hardwood (proper maple, oak, walnut, cherry... not birch or poplar) lip/edge at the front with good joinery (spline, rabbet, T&G, biscuit, tenon, dowel, etc.). A minimum 1-1/2"D x 3/4" wide lip/edge can safely span to 48" but this certainly adds cost and labor to the construction and needs a framed design on the verticals to look appropriate. Sounds like you're using base and crown, so that should work with the style.
I never expect to get actual 12" deep shelving. The industry usually designs them 11" to maximize material after the kerf in ripping a 48" wide piece of plywood into four lengths. Maximizing materials via computer cut diagrams tweaks everything a tiny bit.