I disagree that any of these movies have rendered any of the other films pointless, if only because the point of Star Wars has never really been in its plotting. The plotting is a means to get to the themes, and even the bad Star Wars movies are trying to say something, thematically, within their own runtimes. Sequels don’t nullify or make pointless their predecessors. But I think what this movie was trying to say for itself was so confused and honestly, kind of infantile - it’s just hard to get a good hold on what it wants to be AS a story.
Plot is not important in Star Wars, except in this film plot drives everything (infuriating). But that’s not even the problem. You’re right, SW is about the characters and themes. And it’s that very thing that TROS undermines, not the plot. TFA and TLJ told a story that thematically justified the existence of the ST in the context of the larger saga. TROS completely killed that meaning. Those two films still exist as they are as individual films, of course, and nothing can take that away. But as a cap on the story of the ST as a whole, TROS has rendered the endeavor largely pointless.