slice of life is literally day by day of the characters, again, there is no filler. (and let's not forget that filler by definition is "what it's not from the original source"
You can just say that there was chapters you disliked, and it's expected with more than 400 chapters
Okay, context is important when it comes to understanding language..... so the term filler can usually mean stuff the anime staff adds to just fill the story or series, but it's also stuff that's canonical, but just stretches out a story or series.
For instance, the author making chapters that drag out the points before they are married or what not. Stuff that make the series feel like dragging.... of course, in that context, what can be considered filler is subjective, but it would be filler from that person's perspective.
Yes, Slice of Life is day by day stories of the characters, but there is often something in the story and if there is some plot point, they can still reach that plot point.
It depends on the individual slice of life story. It's more complex than that.