So question is: How to get steam overlay work in discovery when I'm launching it via discovery launchpad connected to Steam?
"In most cases the in-game overlay will be available"
If you ask me, it probably just doesn't work.
I had it working once. I remember it distinctly, entering the game via launchpad and the overlay in-game.
Are you sure?
Well, if anything has changed (OS, hardware etc.) then that might be the issue. If you can remember how you got it to work I guess you wouldn't be asking. Google.