So, my personal experience of the installation is been a bit complicated by the tutorials i've find on YT. Basically I was stuck in the part where after the installation you need to change the directory (or add a new shortcut) for the launcher. Many video didn't explain that or use a previous version of the OS with slightly different interface. In the end i've found this great tutorial online and completed everything.
There is anything i should do (both on the device setting or in the Ubisoft setting) to improve the experience on my Steam Deck?
Btw. The download of the game (AC Origin) was superslow (9mb/s). Any way to fix that?
This is not a good idea. I tried it, and while technically yes it works. It's just not worth it. Here are the steps you glanced over.
How to install SimCity 2000 on Steam from GOG.
Download EXE from GOG
Open the ~/.steam/steam/steamapps/compatdata/ so you can see which random ID the game is given
Open Steam and add a new non-steam game
Add the simcity2000 installer exe
Go into the installer properties and set the game to use Steam Play/Proton
Run the installer
Go through the install process and get the new game's id. for me it was 3594652109
Complete the install and exit
Add the runner exe and the wine directory in the properties
Save your changes and run the game.
In my case this didn't work, mostly because steam can't launch a DOSBox preconfig link while Lutris can. I am certain that if you installed SimCity 3000 or Ubisoft Connect it probably will.
Regardless still not a good idea. While this will get you the same effect as Lutris, it's much more involved and requires more conifguration. At least at this time. I hope Valve adds the functionality to install other games to Steam like in Lutris in the future as it will break down a lot of barriers. But as it is don't do it.