Have a look at the Project Logical Editor. I’m pretty sure it can adjust volume automation for multiple tracks at the same time… In a given range or across the entire timeline.
In that case you’d probably tell it to do 2 things since some tracks won’t have volume automation, raise all volume envelopes by 1.5 db and increase the fader volumes by 1.5 db… or whatever (assuming fader gain is in there, don’t recall for sure). As you said earlier, the latter won’t have any effect on automated tracks, will be over-ridden for those tracks as soon as you start the transport… But it will match the gain change for non-automated tracks.
Something to try anyway. Project Logical Editor.
Just as an FYI, some mix engineers put busses after the automation so they still have manual post-automation control if necessary. That might also be something to consider in the future.