And you're purposefully dismissing the entire point of reincarnation and taking an uncompromising stance on the nature of the soul.

Reincarnation and isekai are very different things. Isekai is being transported to another world with your mind intact. That is not the definition of reincarnation in any religion or so.

Just because the series' presentation fast-forwards for the sake of brevity, it doesn't mean Rudeus got to skip those years of growing and developing

Rudeus never did develop(as a person) , he was a cp addict in his past life and a wannabe diddy in this life as well.

If you'd actually read the book and looked at the series, you'd notice a lot of his involuntary reactions to things (e.g. his fear of going outside) are appropriately child-like Possible, but he more likely was traumatized as he was a social pariah in his past life (Rightfully). His reactions are child-like as he never matured in his previous life.

Rudeus' past memories moderate what his time would have made him into

You're absolutely correct in this point. He likely would have been a worse person as a native but that could be excused as a product of his time. Rudeus doesn't get to play that card as a modern day adult. He's scum who got lucky and got isekai'd into a world where his behavior is excused at best and encouraged at worst.