The Strongest Sage with the Weakest Crest

From what I can tell, people want to label this as a typical overpowered main character with a gaggle of girls isekai. Well, the main character is only interested in one of the girls and only one is interested in him, so definitely not a harem. Yes, the main character is overpowered, but even he sees something wrong with that. He wakes up thousands of years in the future (of his own world, not another, and on purpose) to find that the strengths of humanity have taken a major downfall. He spends his time trying to figure out why this has happened to humanity, how deeply ingrained into human society demons must be to have caused this, and trying to power up those around him in an attempt to restore some of humanity's power. If anything, this is far from the typical isekai where someone lands in another world.