As other members here have already stated in one way or another, one big thing these sequel scores have going for them over the OT and PT entries is the actual development of character themes over an entire trilogy.
It's a lot of fun going back and reading through our discussion threads for TLJ and seeing how, while some were expecting and hoping for Williams to pull a Shore and really advance and build upon the themes he had laid out in TFA, many were, understandably, expecting for Williams to do what he usually did with his sequel scores: revisit old themes through brief and limited renditions (perhaps with the occasional surprising variation here and there) but with largely no development, all while the new thematic material took center stage as the older stuff got relegated to the backseat.
Which, again, was a logical assumption to make! That was the approach Williams used for The Lost World, that's how he handled things with the Potter sequels, that's largely how he handled character themes in his prior Star Wars films. Outside of the scope of the film that saw that theme being introduced, Williams never really seemed to have the desire to advance old themes for characters when there was new music to develop and focus on.
The first time we hear Leia's theme in the OT and the last time we hear it in the OT. It goes out with a super brief rendition. Hardly seems like the last time you will be hearing it in the saga. The first time we hear Han and Leia's theme in the OT, the the last time we hear it in the OT. Nothing really has dramatically changed about it.
The first time we hear Vader's theme in the OT, Vader isn't even on-screen and it's introduced uncharacteristically by woodwinds. It's concluded usage in the OT is in a very similar, albeit much softer, way.
First time we hear Anakin's theme in the PT, last time we hear it in the PT. It's cold and distant in the end, a noticeable departure from how it was introduced. However, with the theme itself having been only a cameo appearance in the last 2 films of the PT, such a departure can't really be characterized as real tangible development, more of a brief alteration to match the character's new head-space as the theme itself isn't even synonymous with Anakin, only his childlike state.
And the list goes on and on. Character themes within Star Wars had largely gotten cool variations here and there but no real conscious development as connected to the characters said themes were attached to.
But with the ST, things are remarkably different. Rey's theme is first heard in TFA in a way that very much hearkens to the idea of someone who is just beginning their journey while also expressing the loneliness and perhaps lost nature that someone currently finds themselves in in searching for greater purpose. We last hear the theme in a way very suggestive of the completion of a journey and of someone who has matured through the events they have experienced. The theme concludes, like the character, in a way far different than how it started. Poe's theme goes through a similar metamorphosis also perfectly in line with his character growth, first heard in a way that perfectly embodies Poe's daring and hot shot nature at the start of the ST before last being heard in a way more befitting now of "Poe the leader" than "Poe the pilot." Those final thematic statements mean something this go around and are utilized by Williams as an opportunity to highlight character change.
And Kylo Ren's theme, perhaps the best example, is introduced as this. It's huge, it's powerful, it's commanding of attention. This is someone to fear. By the end of the very next film, the theme has changed to match the evolved state of the character. His theme has more structured orchestral backing now, akin to the new First Order army he now has complete control over as Supreme Leader. The denser state of the rendition gives the theme more direction and clarity to its statement, similar to the sureness the character has now acquired throughout the film. By the very last film, it concludes by being used like this, this and this. You could not be more removed from the initial usage. It's now light, hopeful, very much a theme for a character of good. Kylo's theme, as a result, may be the most developed theme in all of Star Wars. This is not to dismiss all the wonderful variations of the Force theme we got across the 9 films, nor to ignore the unique permutations that Luke's theme has undergone.
But as far as living up to the true purpose of a character theme, a recurrent musical idea associated with and fully representative of a particular person, Kylo's theme takes the cake. Every significant beat and change in that character feels like it is marked by a concurrent beat or change in his theme.
I'm certainly not trying to critique or criticize one approach to theme usage over the other. Rather, I'm just trying to make the point that it seems like Williams went into this trilogy with the very conscious and deliberate choice to try a different approach with how he dealt with character themes (and sequel scores in general). He seems to treat them with a reverence he had normally forgone when doing scores to sequels in the past, reprising them unabashedly in new ways that connect with how the characters themselves are changing.
As I started out saying, in trying to compare and contrast these 3 new scores over the other two trilogies, one way that this trilogy does stack up undeniably better (IMO) than the other ones is in character thematic development. Had Williams been able to go into these 3 films with a clear roadmap of where each character was going, he could have potentially capitalized on that approach even further, but I think he did a damn fine job despite that.
Sorry for the long-winded post. This is all pretty much stuff that has already been said in one way or another alongside fairly obvious observations that I'm sure most noticed and thought about, but I wanted to verbalize these thoughts nonetheless.