The ending arc could ended in higher note if the shitty father arc cut off completely and replace by an average father figure, keep the biological father a deadbeat.
Introduce future father figure
FF comes to Japan for raids
FF find MC's badge
FF wants to report but siblings fight back
FF, weeb girl and boob girl joins sibling in 5 people party vs wind dragon.
Keep fighting and arguing, FF impart knowledge and train siblings
Deadline incoming, weeb girl and boob girl stalling while FF and siblings fight dragon.
Win vs Wind Dragon.
Serpent emerge, FF hurts bad protecting siblings.
Exit covered by rubble and FF is dying. Can't clear rubble since serpent goes brrrr everywhere.
Siblings vs serpent. Must win before FF turn off forever.
Deus Ex Machina power up, siblings win, got full restore potion.
Clear rubble, exit and happy end. FF open branch to help siblings, boob girl can stay to sell FMC's weapon.