This manga is pretty good to this point.
MC is not completely overpowered. He got strong using past life memories and intense training from childhood, but not to the point of single-handedly defeating the future demon king.
"Power of friendship" doesn't mean "I get individually stronger because I have friends, somehow" but rather "friends compensate for weaknesses".
Killing a major historical figure has consequences. The MC even realizes by himself that his former love might not be anymore.
This is a story that makes sense.