For me this is prequel, the reason why he reincarnated is because he underestimated himself, he say that the time he took to kill outerspace monster are slow but in fact no one able to kill them as fast as him, rather the sequels titles have "world's strongest sage" meaning that even before he...
It was good at first but when the sudden sci-fi come it's become unbearable
Edit: i mean damn who would thought there's a sudden sci-fi plot twist, it was all fantasy there's even no technology but suddenly there's technology and modern city's buildings everywhere