The author overcooked. 1 - Deru and Elf are super strong yet they hid their skills, meaning that Est almost died in the first chapter and no one cared. Deru is super strong because he is the MC that we are supossed to see ourselves as, meaning that he his an excelent teacher, father/brother figure yet the girls also had crush on him, making many of his interactions see as an Older and Stronger adventurer pretending to be weak only to take advantage of the party and become the leader.
2 - The Author didn't plan or didn't know how to write. Deru was against kicking Est out, showing his experience and being a guide, yet after chapter 2 he only acts as a guide for the party and literally paints Est as a villain for being rightfully offended. Many problems on the story hadn't consequences, most of it was Est saying 'Yeah, my bad, you guys are perfect and I was a cry baby that needed to grow up.', this is the solution for all conflicts, Est taking the role as the wrong party.
3- Deru is a overpower MC and has a harem. Yeah, we all know that it would happen. Dragon girl is pretty much clingy on Deru, Elf started respecting and liking Deru and the FMC has a clear crush on Deru, going on dates with him, literally stopping her training and avoiding Est to focus on Deru, like, almost all interactions in the Manga is one of the girls with Deru.
4 - No one from the party learned anything. Yes, they grew up and mature, but they never solved their mistake, Est had to be the bigger person while taking all the blame, which is sick. Deru and Elf almost let Est die at the start because they were selfish and had their own goals, Est proved to not only be a good support, but also good at everything, even positioning and estrategy as we see that the party even had problems in DRINKING POTIONS without Est.
5 - Hypocrisy, this is the name of the game. FMC played the role of tragic heroine and Deru as her guardian, yet no one tried to solve their problem with Est. Think about it, if it wasn't for Est confronting the party in the few times that they met, they would never talk about the issue. The fact that the party hired the dragon girl and decided to try reforming her instead of talking with Est shows that they never cared about him.
6 - Being an adventurer is a job. Est and FMC created the party together and turned it into a famous group only to kick out Est when he "wasn't needed anymore". Est has every right to be mad since not only he lost respect, money and already had rumors about him being useless, but he also had to start from the start since he was "fired" without warning. When Est confronts his former party, he has every right to mock them as he had to deal with bad reputation and being without resource.
7 - They treated Est as the antagonist while trying its best to show the party as heroes. No, the party SHOULD be seen as dumb idiots and selfish, even if they had good intentions it doesn't solve that they made a mess. The reason why many are rightfully mad is that when the Party suffers from karma, is treated as wrong and that they are innocent victims that shouldn't be suffering while trying to paint Est as some spoiled boy that never liked the party and never loved the FMC. The party should have suffered and slowly realized how much they messed up, not only speak about it, but learn and grow. The fact that they couldn't take Est rant shows that they never learned. In a good story, Est would have ranted, Deru would go protective mode and FMC would step foward, cutting Deru and speak with Est, agreeing with Est and telling him that Est was right or something similar, giving Est the first sign that he may have been blowing things out of propotion and slowly Est and FMC could solve their issues, with the story pointing out that Deru only causing more damage by intruding at the fight only to protect FMC and berate Est as he shows clear favoritism towards girls in the story and the FMC.
8 - Deru soloing the demon is terrible, throw away all the "Lessons" and end up as another power fantasy, I really hated how the party was mad that Est also gained fame for defeating the Demon despite the fact that, 1 - Est party is clearly stronger than them and working together they had a change to defeat the Demon, 2 - Est is the reason why Stardust is alive as Deru wouldn't be able to fight while protecting the party.