95% being trial and error. At least where I work.
soulsource
The Orange Pi Neo will ship with a custom version of Manjaro, and is imho the only Steam Deck competitor that is even worth considering.
Just take an Xbox 360 gamepad, and an Xbox Series gamepad in your hands and compare them. Press the buttons, move the sticks, try the triggers.
One feels like quality. The other feels like - especially the D-Pad.
I loved that game, and completed it twice, but the last chapter (or last 2 chapters - depending on which ending you get) is super annoying. The encounters are repetitive, and there are quite a lot of them. It's almost the same group of enemies again, and again, and again. Once you have a working strategy those encounters aren't even that challenging, but if you play turn-based, they take a lot of time...
I'm still hooked on Backpack Battles. It's slow enough that medicine-induced-brain-fog ridden me can play it, and it's a lot of fun.
Stay away from any Xbox Series X/S gamepads. They are cheaply made trash.
I bought the standard version of it for my Deck, expecting it to be somewhat comparable to my Xbox 360 gamepad (which I really like, but which does not have Bluetooth), but nope, it is so much worse...
A friend bought the Elite version, and he also agrees that those gamepads are utter garbage.
Mesa has its own OpenCL implementations for AMD GPUs too: Clover and RustiCL. However, Clover is not really developed any more (afaik) and lacks some important extensions, such that many programs can't use it. RustiCL is rather new, and I don't know how well it works.
While technically you don't need an account to access Play Store (e.g. via Aurora Store), the Austrian eID service ("ID Austria") is only available there. This is true for both apps, the "Digitales Amt" and "A-Trust Signatur".
When I got my Steam Deck the official dock did not exist yet, so I went for a relatively cheap USB hub with power delivery and display port alt mode. I ended up buying this one, and haven't regretted it since: https://www.dlink.com/de/de/products/dub-m420-4-in-1-usb-c-hub-with-hdmi-and-power-delivery
You can also use Steam as a launcher. In Desktop Mode there is a menu entry "Add a Non-Steam game to my Steam Library". For Windows games, you can just browse to their .exe file. After adding it to the library, you can open the Library Entry's Properties page, and choose Proton as compatibility tool.
That way you get your non-Steam games in your Gaming Mode launcher.
To get nicer images, there's a website named https://www.steamgriddb.com/ that also has a small Flatpak tool that you can use in Desktop Mode to set icons/banners for your Non-Steam games.