Yeah it's confusing that this is called just "side stories" even though it's the sequel series. Some publication shenanigans responsible for that?
The first chapters will feel less polished after reading these more recent chapters but it still has some good stuff in it.
From my understanding, it's related to a change in publication that happened mid-series and how that new publication decided to handle the continuation of the series. It's not really a sequel series, just a direct continuation but on a new publication website. The "side stories" naming isn't official, I don't think. It's all corrected in the volume releases. This stuff is basically volume 5 onwards (chapter 94 onwards). So the story really has 139+ chapters.
It mix up with the main story for the first couples "side stories", then past a certain volume, it's just straight up only the side stories. For what i've seen of the french publication.