And yes you’re right Vader leaving Bespin and arriving on the Star Destroyer definitely could have been executed in the original film so the motivation for the change is unclear.
They were running out of time on ESB and Lucas was only concerned about getting things done. It’s also true that he doesn’t really like the film, or at least thinks it’s the worst Star Wars movie, probably because it was the one that he had the least creative input into.
Sure, Lucas was still a big part of Empire, but Kersh and Kurtz were more in control of making the movie than anyone besides George has ever been in any of the non-Disney movies, and it shows.
When it comes to the most important creative aspects like story, screenplay, concepts, final cut, etc. Lucas pretty much had the same level of input/control as in the other two films. The only aspect that Kershner and Kurtz were actually in control was principal photography, until it spiralled out of control (i.e. went seriously over schedule/budget).