I found this on Ao3 and recently found out you mainly upload here, so I'll be regularly reading this fan fiction here. Anyhow, to my review. I will put a summary at the very bottom. Of course, this is also just all my biased opinion, so feel free to agree or disagree.
Off topic: Firstly, when I first started reading this back in Ao3 (Archive of Our Own) I was kind of put off by the very first chapter, and decided to put it on my backlog, however, in the last few weeks I have gone through and read it all (Including the newest chapters here since on Ao3 only the end of Volume 4 has been uploaded.)
HEAVY SPOILERS AHEAD, PROCEED WITH CAUTION.
Long: As I mentioned in my off topic section, I felt off reading this since the very first chapter had a seemingly OP MC, and that's been done a hundred times over, however, it's written well, and that kept me going. So, onto chapter 2 and it's a flashback that explained the 1-week time skip which I liked, and introduced many other characters and things. What worried me the most was how the system was going to be handled and if it were going to be riddled with just status sheets and skill sheets, anyway, just stats pages to fill up word count, but by chapter 3 most of the functions were removed because (spoiler) the MC evolved.
However, the stats page was still being used albeit not very frequently (And by around Volume 2/3 it's barely used and only seen sparingly.) Now reading a more detailed and revised synopsis tells us a little more on what this is about, also, chapter 1 gives us a snippet of what to expect (World building, romance, slice of life, harem and so on.)
The MC, from Volume 1 seemed perfect at the start. However, cracks early on start to appear as he didn't utilize many of his abilities, and then he starts to hear voices and that made it an even bigger mystery for us as readers which I personally liked. However, by Volume 2 and onwards (Especially Volume 3) the MC starts to grow and improve, it isn't in a stupidly fast rate, and as we see in Volume 3 and at the end of Volume 4, he sometimes doubts his own decisions and actions (Because of the events at the start of Volume 3 which force him to mature and grow.)
Watching Jin (MC) grow through the entirety of Volume 3 and the 1st part of his character arc at the end of Volume 4 was wonderful. In Elsha or Rudra's words: He needed to learn to forgive himself for what had happened at the start of Volume 3, accept what happened, grow and love himself again.
And yes, his first character arc/development phase was just that. Acceptance, and self-love. From the small flashbacks, dream sequences we saw about Jin's personal past, he had stopped growing and maturing before his untimely death, however, upon reincarnating in DXD, he was given that opportunity and he has done wonderfully in that aspect.
The people he meets, the friends he made and the lovers he has, they supported him through it, unlike in his past where his parents were emotionally absent, and his one friend abandoned him because it got a bit too much (Although it's a very realistic reason, most of us are not equipped to help someone through trauma.). Here in DXD, he has a great emotional support group.
I PERSONALLY think the second and probably final part of Jin's character development is... Moving forward, completely, and breaking away from his old world or self. In most of Volume 5's storyline, the author has shown many side characters moving forward in their lives with the help of Jin or his lovers, or with the help of both of them. (Kurousagi with her own insecurities and doubts, Sekhmet with her husband, and recently, Aine, Rayanre and Yasaka.) Jin has been there and seen them moving forward in life.
I can't wait to see the conclusion to his second arc, I will say that it will be definitely be emotional like his first arc (His fight with Sairaorg was definitely emotional, especially his [Queen] chant. It was great.)
I read your review, and agree the first 3 girls joined quite quickly, however, the rest of the current harem joins him at a steady pace and with good development between them. I especially like what happened to Yasaka in Volume 5.
Now the final part is world building. Honestly, I love it. DXD has been expanded upon a lot, especially the pantheons and factions. Jin still enjoys his school life to the fullest with even some minor characters (His two friends Najima and Kouta.) On the way to getting girlfriends themselves. Anyway, a lot of detail and lore went into the pantheons and factions themselves, so I'm looking forward to more of it in the future. And especially how you will write the future Tensura arc.
Short: TLDR:
Writing Quality: ⭐⭐⭐⭐⭐
Not many complaints here, the occasional typos, but nothing too serious.
Updates: ⭐⭐⭐⭐⭐
I mean, it's being updated, there's not much to complain about.
Story Development: ⭐⭐⭐⭐⭐
I put it in detail, but personally, I love it. A great mix of Canon and Original plot.
Character Design: ⭐⭐⭐⭐⭐
One of the best OCs I've read about. His growth has been great to follow.
World Background: ⭐⭐⭐⭐⭐
It's High School DXD and it's being expanded upon, and a lot, I mean, it's great. No complaints at all.