I think this is the problem with moving away from making content that players can miss. I would have preferred that we were given two options(join the shining blade/don’t join the shining blade) similar to how things were done in the original personal story. A lot of stuff has seemed “out of character” for my main ever since they stopped letting us make choices in the story.
My head cannon is that my main(an Asura) has very questionable ethics and would have no problems with making an oath to a human organization that he didn’t really care about or intend to keep should it interfere with research. I would have preferred to reject the offer to join the shining blade and have a more antagonistic relationship with them throughout the story(while still working with them due to a common enemy.)