JP authors are really milking this 'expelled-from-hero-party' trope dry, aren't they?
MC gets kicked because incompetent(?). Some rando with no proven track record is hired to replace him.
I can already predict the future chapters. The useless leader gets angry at failures, thinks it's the MC's fault for some ungodly reason, and goes to get revenge.
At least the dungeon building is a nice change of pace.