While I am not aware of any way to run custom software on the Steam Deck while it is on standby, you can drastically reduce the power drain if you shut it down fully instead of leaving it on standby. You can either use the “Power” menu after pressing the steam key, or long-press the power button to get the option to do so.

Also, if I am allowed to do some absolutely shameless self promotion: Bus Simulator 21 is verified, and 18 is playable.