Would just like to make sure I understand; when you say ghost shifting, the behavior you are describing is that, under high torque such as steep climbing or out of the saddle efforts, the bike suddenly shifts to a different gear on its own accord, correct? Not skipping, but in a new gear. Is that new gear a higher or lower gear? Does it happen more often in certain gears than others? For instance, if you are climbing, I would expect you to be in a low gear. Does the bike then shift to a higher gear/smaller cog? Does your gear indicator change at all when this happens? Can the stoker see the chain on a different cog than you selected, or even a different chain wheel? And the bike then shifts back to the originally selected gear when you let up on pedal pressure, correct?
I’m mystified why this would happen with a wireless derailleur vs a cable actuated derailleur. Did the chain line change significantly when you made the "upgrade"?
Sorry, none of this is helpful, but curious minds are wondering...