Well, with a long series of books, most of them self-contained, it's inevitabile that many seem "skippable".
Also because they are narrated by a different character, so every time it start from a different POV, a different situation that usually is not connected to the previous...the exception is David trilogy or, in a more vague way, 32 and 33.
Then I would be more cautious in defining them "fillers".
The first 8 books are necessary to explain all the main characters. Book #2 may seem unuseful but it shows other morphs, the life of a Controller, and some group dynamics. And Rachel's family, and, of course, first book narrated by her.
IMHO, the very "filler" or unuseful books are 24-39-42...in conclusion Helmacrons
. Other books are in fact useless for the plot, e.g. #11 The Forgotten, but it's quite interesting and introduces Sario rip.
Also #32 is avoidabile, but ignoring the absurd story of the two halves, this book talk about the Anti-Morphing Ray that will be important for the next book.