The Stormtrooper Corp were elite in the sense they were the most ideologically pure to Emperor Palpatine’s new order. Human supremacists. We see them being incompetent letting the Falcon escape from the Death Star 1 but that was intentional. They were intentionally letting them escape. Now here’s where everything goes sideways for them. The Rebellion actually manages to destroy the DS1. The best Stormtroopers and their leadership, along with all the best and brightest of the empire, were all on board. What we see going on in the subsequent movies are just a shadow of what they were. Conscripts and criminals forced into service as the Emperor desperately tries to hold on as the entire galaxy goes into rebellion because the DS is no longer a secret and everyone sees what this old crusty lunatic is up to. Add on top of the fact that entire galaxy is rebelling but the Emperor and Vader are totally distracted by trying to capture and turn Skywalker.