Isekai de Kojiin wo Hiraita kedo, Naze ka Darehitori Sudatou to Shinai Ken
The lazy prince becomes a genius
I'm having difficulty deciding which genre this falls into time travel, reincarnation are meeting the ancestors cause old hom dude that died with sword in hand while training, his youth version looks exactly like him with longer hair. So is him meeting his future self, who died? In his dream? Merged Together, or it could be him meeting his ancestor to fault demons way back when, since the cat mentioned demon shotgun shells on 150 years ago, and dude has a grudge against demons. That his mystic arts was dormant inside him and then we fell asleep it put him in touch with his ancestors. That's quite a stretch, but it's mystic arts it's mysterious. I could be just thinking to much into this to, but I'm confused just as well as hom dude I mean, why does old dude keep popping up in his dreams, then let the old dude was looking at him freaky, so there has to be like a familiar connection.
Kyuutei Mahou-shi Kubi ni Nattande,Inaka ni Kaette Mahou-ka no Sensei ni Narimasu
Point Gifter Keikenchi Bunpai Nouryokusha no Isekai Saikyou Solo Life
Isekai Mahou Wa Okureteru! (novel)