Generally I think far too much mouse-clicking is needed when working on server actions and extra keyboard shortcuts would make a big improvement in efficency etc. However, switching output on/off is not something I find too much of a problem; you don’t need to scroll as you describe - you can use the button here:
(thought a shortcut key would be welcome).
I agree the spacebar could be put to good use - eg to respond to the current step (whatever it is) in the most useful/expected way: jumping to the text input for a comment step or to open the relevant options panel for a database action etc.