If you're in Canada, this is the one I got. Similar to that, but it has ethernet, 3.5mm audio, and VGA on top of what the other one has. I've also tested it, and it does allow charging via the hub's USB-C port.
https://www.primecables.ca/p-398125-cab-bs-hc101a-10port-usb-c-hub
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.
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.
Kinda. They still work as taps, so if you release them and touch them, the device registers a tap. But for holding (for instance, using gyro) I wouldn't recommend it because the input fluctuates a lot.
Note for others reading this (not applicable to OP) but glass protectors aren't recommended for the 512GB model. It clashes with the anti-glare coating on the screen and causing the picture to become fuzzy.
If you're on a 64/256 though definitely consider investing in a matte glass screen protector
Xbox controllers are just awesome with it and are really solid bluetooth-wise. I got a bunch while they were on sale for xmas so that my kids and I could all play 4 player couch co-op games on the tv. It's been a blast.
I'd tried the same with a rpi and some janky set of various controllers and it just flopped and was 95% spent troubleshooting. Once I standardized and get them setup it's been silky smooth and robust as hell even with new updates.
A folding bluetooth keyboard-trackpad combo has been a lifesaver for me! Any time I need to work on something in desktop mode it's saved a lot of time and frustration.
I usually just use the built in track pad and the digital keyboard (or whatever you call it) but sometimes is it easier to just type with a normal keyboard if I have to change a lot of config files (I use a USB hub and keyboard then).
But I only mod my games in desktop mode so maybe that is why I don't see the need of a true dedicated keyboard and mouse set up?