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 used to be able to set the mouse speed on the windows operating system. Sometimes the software you are using overrides this, so maybe check your software settings to see if they made that possible.
I am running Steam OS on my Steam deck, not windows.
There is no input speed control. Plus I would like something Steam Deck wide and to not have to adjust it in each game.