You can buy all sorts of stuff if you have money burning in your pocket, but as far as essential goes, you've already got it all.
That said, you might want to get a multipurpose USB-C dongle, for connecting the deck to an external display, keyboard, mouse, ethernet and such. The Deck Dock is okay, but there are cheaper options with the same functionality.
I bought one Dock off Amazon (it may have been JSAUX but I think it was an even cheaper option) and it's great if I just want it once in a month or do for downloading mods and stuff, it's the only addon I bought so far and I doubt I'll need more anytime soon
Same here - bought off Amazon. Usually it's used as a glorified deck stand, but when I do want to casually game with a bigger screen, it comes in handy. The USB ports on the back are great too for hooking up external controllers.