I'm finally getting my first Steam Deck with the 512 OLED. What accessories would you consider imperative?

i'd say a usb hub, so you can easily connect kb and mouse if you want to get into desktop mode.
the one i have has usb3 in and out for power passthrough.
a couple of normal usbs , hdmi output, and ethernet port.

for me i also use that hub to add in usb storage (mostly if i want movies when travelling or something)

i use the ethernet port for large downloads - my wifi is a bit lame.

i'm thinking about a 3d printed stand for it (that i can clip the hub into) - but i might just make one out of scrap wood because it's nicer than shitty plastic.

i have a dock and a steam controller, and it's performing nicely as a pc attached to the tv as well as a gaming console/handheld

None are necessary. Buy what you need, not what other people say you should get.

For 9/10 people Steam Deck is a complete package.

maaaaybe the dock (my friend is telling me to get one)

Unless you have something to replace its functionality with already, just get the dock. You are probably going to want to plug in a mouse and keyboard and monitor eventually; it's a full x86-64 linux computer with a desktop mode in addition to being a hand-held gaming console.

Personally, I usually use it more like a traditional gaming console (with my old xbox360 wireless controller plugged into the dock and hooked up to my monitor) -- that I can pick up and take with me occasionally -- but have written code on it and installed mods/patches that have to be copied into specific parts of a game's folder and such. Just being able to use it like a regular computer is nice.

I thought the case it came with was fine personally, but I've only traveled with it a little bit.

If you aren't sure about a dock, you should at least look at getting an inexpensive usb hub. I found that and a 3d-printed stand was good enough for propping it up to use with kb/m on occasion.

Other than that, for me at least, installing Anydesk made things much easier to set things up in desktop mode and transfer files from my desktop pc.

for me dock and skins to protect. The screen skin is just a film but the body type protector is a rubbery type stuff but mine came with an adapter so it would work fine with the steam dock. My steam dock has an annoying issue with the power where it sometimes just does not work so if I had to do again I would get a third party dock. The worst part of the issue is that its just not quite bad enough to go through the hassle of doing a return but just annoying enough to come across it regularly to remind me to be annoyed about it.

A dock is really nice, the one by Valve isn’t exactly necessary, I bought one for 40€ on Amazon and it works perfectly.

About the dock, I would add that if you want to do emulation stuff, a dock with kb/mouse makes it a lot easier. Can you do it with the trackpad and on screen keyboard, yes. Will you hate yourself for trying after 5 minutes, also yes. 😂 Maybe it was just me, I found doing anything other than basic settings changes nearly impossible without kb/mouse.