I'm not a fan of flashbacks either, but they were the least of Dallas' problems in season 12. The idea of a movie about JR was actually a good idea from the writers. Sue Ellen already had her revenge in season 11 when she prevented JR from taking over Weststar and of course she had to get another revenge after Nicholas' death. The idea of a movie was something different and made sense in terms of JR's reputation. The beginning of the storyline was particularly promising, when Lucy and Jeremy were still involved. Lucy could have suggested her mother, the famous author, as the screenwriter and we would have had a nice visit from Valene.
I would have preferred if Sue Ellen had realized during the actual shoot that it wasn't worth it, not just because it would hurt John Ross once he watched the movie, but also because of herself. She should have realized that JR isn't worth getting any more attention and should instead focused on the future, be it with Don or whoever.