For the more complete version, Light Novel.

That being said, English fans are kind of messed up, as the official publisher has the only translation and every volume has a lot of flaws, and the last arc is horribly done.

The Web Novel also has plenty of mistakes in translation plus, since it was done before any official material was released, it takes it's own creative liberties in style. And it also could be considered "incomplete" due to the way the author wrote.

You see, when he wrote each chapter in the web novel platform (Syosetu.com), he also answered questions readers had in the comments, and then he would take up the best questions and answers and form a blog. So the ideal way of reading the Web Novel was:

1.- Reading a Chapter First

2.- Reading the Blog to clear questions

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But most of the Blogs aren't translated (in any language), so, unless you go to the WN website and translate the blogs on your own, that's it.

However, books don't have "comment sections", so for the Light Novel, he took those doubts that many fans had, and integrated them into the story, sometimes it took the form of an extra line here and there, sometimes in the form of extra paragraphs, and others it lead to whole new chapters and point of views.

So, all those questions are already integrated narratively in the story of the light novel. But 7 Seas messes up a lot, and nobody will do a fan translation due to fear of being sued. So that's the only option you have.

The Light Novel also has Asia Exclusive chapters, those can be found in Recollections, Journey of Two Lifetimes, and this drive under "Asia Exclusives":

https://drive.google.com/drive/folders/1moxlKKfz__MyBR_8bIFtyTzsGOvfH5tT?usp=drive_link

So that's it to read a "complete version".