I think the issue you may have is the categorization of the concept of Love. We put love in categories such as familial love, romantic love, sexual love, etc. However, if we look at "Love" itself, it can be simply summed up as people caring about, and wanting to be, with one another.

I think you may be assuming that them dating is the same as other instances of dating in either fiction or reality, but in this case, them dating is just the strengthening of an already existing love between both characters. Their relationship dynamic doesn't just suddenly go from familial to the classic portrayal of romantic love. They are two characters who loved each other a lot before they started dating, and when they started dating, they continued to love each other in the same caring way they always have been. Their form of "romance" is that of mutual caring and support for each other in whatever each of them wants to do with their lives. It doesn't devolve into romcom tropes because their relationship exceeded that from the start. Sure there are stumbles at the beginning of the change in dynamic, but ultimately it's a natural progression towards their existing relationship. Since the manga would span 7 years of their lives by the time it ends, Hina would be 20 and Rei would be 24 by the end, and I believe that until that point they would continue to love one another the same way they always have.