The term “filler episode” is blatantly misused in fandom spaces and it's really goddamn annoying.
In the book "Manga and Anime Go to Hollywood: The Amazing Rapidly Evolving Relationship Between Hollywood and Japanese Animation, Manga, Television, and Film," by Northrop Davis, Davis explains the that concept of a “filler episode” in animation comes to us from anime adaptations of manga. Anime is frequently produced at a faster rate than manga, which means that anime adaptations of manga often have to come up with their own “filler” stories to slow down the pacing of the main narrative, because the manga isn’t finished yet.
"Filler" does not mean "low-stakes episode that doesn't progress the main plot." Filler does not mean "character moments that you did not like." "Filler" does not mean "anything that isn't breakneck-paced main plot."
It's a specific term and you are misusing it.
[ID: A screenshot from Futurama showing a scene where Morbo looks at the news camera and yells, "Windmills do not work that way! Good night!" The subtitle has been crudely edited to say, "Filler episodes do not work that way! Good night!" End ID.]