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

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.

Lately Microsoft has been friendly with Linux. They even released a guide in their website on how to install Linux alongside Windows. If anyone is guilty of actively trying to fight against Linux is Epic. For now, at least.

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

I forgot most of my simulation subject, but we did a 2D particle simulator and yes, you will need Navier-Stokes if you want the effect to be physically correct. You will need to solve a numerical problem each frame to compute turbulences. Wind is a force with a source that will propagate depending on its parameters (like viscosity)

If you don't implement that, you won't be able to implement things like walls that block wind and such, which I guess is something you might had in mind. Also, having a changing environment is probably pretty hard, so prepare yourself.

I just took a look at the source code of The Powder Toy, and the class Air (src/simulation/Air.hpp/cpp) seems to implement what you need. Sadly it is not very well documented, so you'd have to work out the relation between code and physical formulation.

Sometimes physically incorrect simulations are easier to implement, have lighter computations, give the designer more control of the game, and makes the player able to better predict how the game will behave and have more fun.