But is it "(A Steam Deck), or (a PC running in Steam desktop mode)" or is it "(A Steam Deck, or a PC) (running in Steam desktop mode)"? In any case both cases include the Steam Deck as a capable device, it would only be a matter of changing to desktop mode

My router is a bit old and its 5GHz performance seems to be rather lacking. I still think the main problem is I'm not testing with a big game. I'll try and post later something.

According to the documentation, you should be able to https://help.steampowered.com/en/faqs/view/46BD-6BA8-B012-CE43. Unfortunately, I don't have big enough games to test, and I think it detects that my internet bandwith is higher than the connection to the Deck

It's probably not worth the risk implied with second hand vs the discounted new unit directly from Valve. You should lower it or just assume you're not going to sell it

I play Lethal League Blaze (previous game from same devs) with my friends a lot, it's super fun, check it out

There was Bibliogram but it closed as it was impossible to keep up with Meta's barriers, sadly

You can install Bazzite and you will have the same experience you would get with SteamOS. Don't use the extra features and you won't be able to tell the difference. It is a "just works" experience with minimal setup. The setup even includes emulators and launchers like the FFXIV one so it is even more minimal than SteamOS.

It may not work directly on a given PC or a specific device, but that would also happen if Valve released a generic SteamOS.

Edit: typo

SteamOS is a less flexible distribution of Arch Linux. You can do the same and even more with other distributions. Even on the Steam Deck.

SteamOS uses inmutable partitions, you can't install things using pacman safely. For OP's problem my first thought is using a virtual machine or a whole different OS. Maybe there are fonts in Discover/Flathub, which is what SteamOS is designed to use, but I don't know if that could work.

OP says the kid has been "all over scratch" so we can assume they need something more

It's another Chromium/Electron based app, so it may not be suitable for low end systems, but still, it's weird that it doesn't load at all. I don't know how to help, sorry.

I find it pretty inefficient to start up a Python environment, a web server and a web browser to do something that you could do with Notepad or even in a terminal. I would use Notepad++, gedit, Kate, vim, vscodium... There are plugins for vscodium (and vscode obviously) to preview Markdown if that's what you need