The steam deck is not limited to just steam games. I have lots of non-steam games running on my steam deck just fine. Steam Proton will run *anything* in linux, not just steam stuff. All you have to do is go into the steam menu and add your stuff as a non-steam game, and it'll work, complete with steam overlay and gamepad support.
Aside from the usual non-steam stuff you'd expect like GoG and EA and such, you can also run windows-only indie games on the steam deck just fine. Like, I put Another Metroid 2 Remake on my Steam Deck no problem. And what's funny is it was actually even easier to just add the windows version of AM2R instead of going through the trouble of installing the linux version!
I have a dock for my steam deck that does this. I got one early so it's pretty no-frills (it adds an ethernet port, 3 USB ports, and 2 video out ports - 1 HDMI and 1 DisplayPort), but they have some 3rd party ones that even embed power bricks into the dock so your dock acts as a second battery for the deck. Neat stuff.
Also, if you go this route, I'd recommend buying an 8bitdo controller. The Xbox controllers connect through bluetooth and constantly drop connections in my experience. You can get around this by buying an Xbox controller dongle, which lets you connect the controller through microsoft's proprietary wireless connection instead of bluetooth and it becomes rock solid, but the downside is you have to install a driver called xone on your steam deck. And that involves quite a bit lof linux that I don't think the average person would be down for. As in, you have to go to the desktop mode, and download the driver source from github and build it from scratch using make. And before you do all that, you have to unlock the steam deck (valve locks down the rootfs in linux so the average person doesn't accidentally fuck it up and brick their deck), and every time the deck updates, you have to do the entire process again. Which brings me back to the 8bitdo controller -- it looks and feels almost exactly like the xbox controller, except it has extra buttons on the back, and hall-effect joysticks so they never drift, and it comes with it's own dongle for the dock that they made specifically to work with the steam deck, so its all plug and play. Can't recommend the 8bitdo controller enough.