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.
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.