I really like this series overall, it's of pretty high quality compared to some others, and the plot points and characters are interesting, but there is one big flaw with it that I do not like, and it shares this flaw with quite a few other isekai.
And that flaw is, it doesn't actually need to be an isekai. He uses zero knowledge from his life in 'our' world. All of his value and strength and ability comes from the thousand years of training in the hall of heroes. And that's fine, that's great, that's a good story. But it did not need to be an isekai. This story could have started exactly the same way it did, and played out exactly the same way it has, without the isekai part.
There are other isekai that are guilty of this, stories that just incorporate the isekai aspect of it without ever actually needing it. And that honestly kind of bothers me, because to be completely honest, most of those stories, this one included, would be better off if they WEREN'T isekai. It's an unnecessary and pointless detail. Sometimes I get the feeling like writers are just using the isekai as a chapter 1 or prologue to their story because it's easy or because they don't feel confident in having their own opening. They should be more confident in their own openings. Hell, have it be a cold opening or an in medias res, those are fine, not enough stories use them, they work great. Not everything has to start out as an isekai.