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.
If you don't mind cracking it open, I'd recommend picking up a bigger SSD. I swapped my 64 GB for a 1TB and it's been night and day.
Also, look into something like Deckmate. You can buy it from them or get the 3D files for a donation and it allows you to mount things to the Steam Deck. Their kickstand even doubles as an 2x microSD card holder.
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.