I have not found a setting within the Steam mode to change input speed. I did boot into the desktop and slow down the mouse cursor there. But it does not seem to carry over into game mode.
The game I am playing does not have a cursor speed setting either.
You should be able to do it with steam input
The only place I found anything about steam input was in the properties of the game and then under controller, I can set it to Steam input, but I do not see where I can make any changes to Steam input, and I am not using a controller but a keyboard and mouse.
Once you enable it, you can edit literally anything about your controller setup for that game… There should be an “edit layout” button.
Here, this should help: https://www.reddit.com/r/SteamDeck/comments/woy7lp/steam_input_basic_guide/
The amount of shit you can do is actually kind of crazy, though you don’t need to get that deep.