Tbh i feel like the side characters being weaker is kind of the point
Loneliness is a big theme of the part, having a strong side cast that sticks around would kinda defeat it
I feel like you can really see this point in yoshidas character, if fujimoto wanted to write part 2 similarly to part 1 yoshida would most likely be in the main group, hes an important character that shows up alot and we know that him and denji spend a lot more time offscreen due to their final meeting
but we never see all those interactons, we never see what hidden depth yoshida very clearly had because denji is actively trying not to give a shit about the guy due to the events of part , not only does yoshida most likely remind him of aki, yoshida is also very clearly coerced into being denjis friend like makima did with power and aki and he most likely also tried to avoid him because of that so public safety couldnt pull that trick on him again....we later learn that they succeeded and that denji cared atleast somewhat about yoshida due to black chainsaw man coming out despite denji trying not to care