Well, I would say modernism and postmodernism are opposing philosophies, much like capitalism, and socialism, and so they clash by default. I would also say RJ quite deliberately set out to create a work, that clashes with past perceptions in a great many ways. My interpretation of TLJ is, that it first rejects and deconstructs the concepts of legends, and heroism, as presented in the first 7 parts of the story, and then reframes it in a postmodern context by the end. I think this clashing of opposing views, is at the heart of the fan division, where many fans view the film as refreshing, and a necessary step in the future development of the franchise, whereas others view it as a betrayal of what came before. For this reason, even if I dislike the direction chosen by RJ, I still believe TLJ is one of the most interesting Star Wars films, and blockbusters in general to date.
I don’t have the time for a lengthy response right now but I don’t think modernism and postmodernism are exclusively opposing philosophies in general (both ideas can exist within a single work) and certainly not in the context of those two films, nor do I think the two films specifically align with those two movements (I actually don’t think TLJ is a very good example of a postmodern work). Even back to my comparison, I think one could easily make a similar argument about SW and TESB. Point ultimately being, such an analysis is subjective, and the idea that the two films objectively clash with each other is silly, as is the suggestion that anyone who disagrees is doing so in bad faith.