I’d say you have the essentials already after getting your 2TB drive. With that kind of storage you won’t have to think about buying an SD card for a while (if ever). You might want to think about getting a case though. Preferably one with a kickstand. Mine also helped with the steam decks plastic shell being misaligned a bit leading to hard edges pressing into my palms. Other than that I’d say most of the (more) essential stuff is software related. Like getting decky loader and exploring its plugins.