One technique is to dock some of your fingers against known reference points such as the corner of the screen, then you can stabilize the finger pressing the button and rely partially on muscle memory to locate the target button.

Yeah, this is the way. Still much less comfortable than a physical button, but not a great issue for power users either.

or best of both worlds and have physical soft keys down the side like oscilloscopes and other test equipment have.

Yes, that's the downside.