God_Of_Hate
I recommend One Piece very strongly. It might be short but easier to understand and easy to get strong there. Also you can learn how to train your spiritual power without using your physical and magic senses. The so called haki is probably soul power. It is very useful for your main character. Beside even though your main character are very strong physically, there might be challenges for him like Kaido, Whitebeard, Im-sama or other strong creatures in Sea. Also there’s so many different devil fruits. I recommend your main character to fight them and learn from them in a fight to improve his own magic. Since you said Akihito has all magic elements, he should learn from his enemies or allies that has Logic Devil Fruit in One Piece. If it is not One Piece we are seeing next world then i don’t know lol. One Piece would defo help your main character to insane degree with fighting, not training. It’s a defo major step for your main character and help until he goes to DC or Marvel someday. Remember it was just a suggestion and good luck.
Uhm soul land, HOTD, jujutsu kaisen tbh I can't think of anyone I sent really mind what world he goes to just hope he doesn't become another low-key is life mc (like when he has strength to do anything yet he wants to be low-key or act like a any infont of other people so he can't live without being botherd..So yeah anyone world is fine just hope he isn't beta mc neither a mc who follows a world's rules like, being in hotd and acting like a human or being in soul land and acting weak because his a "kid"