Personally, I think these flashback chapters shouldn't be done like this. The manga should have started out with a handful of chapters showing his mistreatment from his family growing up, to how he ended up with his party, and the ultimate fight with the demon king. Think that's better than going into a flashback arc just 8 chapters into it. Kind of ruins the pacing and pisses people off more because of how long we'll be stuck in a flashback. People would feel less annoyed over it if it just started chronologically.