I think the best way to incorporate the force ghosts is to reduce the opacity and have them only blink into and out of existence with every lightning strike in the room. They can probably recede back into nothingness for half a second after the lightning is gone (replicating a sort of after-image effect) but I think relying on an audience’s ability to resolve visual information very quickly will help out a lot.

You can also probably get away with using stills of jedi - thus increasing the ability to exponentially increase with each lightning strike the number of jedi showing up in the background (probably in the far background, too) to support Rey in ghost form.