Seinen is aimed towards older teenagers and young adults so it's an absolutely perfect description of this Manga. It's adult themes that are made easy to understand for younger people.
Idk, It feels kinda childish ngl. Probably just not what I expected, but dude being retired, adventuring while tasting delicious food and doing justice for the sake of it, being targetted because he's strong, wasting his money on an assassin with a comical lack of emotion cause he only knows fighting trope that tried to kill him for no reason feels too jovial