I think you'd be better off just having a controller hook up and that triggers a bash script to start steam in big picture mode or lutris.

You probably could do something like this if you really wanted to but this would be clunky. If you're always turning off the machine after you're done using it, maybe it'd be OK but IMO, it would make more sense to just use KDE plasma and script something to get the functionality that you want.

You wouldn't have to worry about booting things up or shutting them down if you wanna switch between gaming and whatever else, you could easily make changes to it, and it'd likely be less complicated.

Hell even a shortcut that opens it through a button combo or something would work for this. Pair controller, hit a key on your keyboard, boom, steam opens. That can be done through KDE natively through the shortcuts settings. Very easy to setup and something I use a lot.