I've never had a problem with MCs not being self inserts, but I don't feel strongly about whatever style is used. Generally when I pay a VN I approach it almost as if reading a book. I'm there to experience a narrative, even if I can influence the way it will go to some extent.
Stablished characters that leave little open for the player to fill in the gaps never stopped me from putting myself on their shoes if I really wanted to. Sometimes just experiencing what a character sees and feels through a first person view serves the narrative better and, if that isn't to my satisfaction, I can always imagine how I'd handle scenarios if it was actually me. In the cases they are a whole character of their own with little input from the player, I do find it a bit more interesting to read along and trying to figure out what makes them tick.
I feel like I'm rambling a bit... When it comes to this VN in particular I really like how the MC has been handled. At some times I might have wished he wasn't so tight lipped, but he has strong enough traits, is very competent, kind and smart while still being "vague" enough that players can fill in the blanks if they feel like it. If fleshing out the MC or giving players more options to make him more to their liking is something that you guys want and/or are able to do, that's all well and good, but I'm pretty happy with the way thing are right now. It would also be a shame if implementing these sort of changes to the MC get in the way of the current development flow. If fleshing out the MC or giving players more agency for choosing personality traits gets in the way of narrative development, turns out to be too much effort to retroactively implement or ends up amounting to just being some fluff, that is a trade off I wouldn't be willing to make.
tldr: I'm happy with MC as is but I wouldn't mind if the way you characterize him changed, as long as the devs feel it's worth it and it doesn't get in the way of other things :)