I'm surprised nobody mentioned this, but chap immediately made me think of:
Futago no Ane ga Miko, where the MC's sister "stole" her identity as the saint and dumped her off to die.
I flipped through the chapters, and it's distinctly more different, much more focused on "revenge against the evil family" than focus on the MC having an actually good life. <_<
Other than that: I honestly don't really see the benefit of having this being "isekai". The MC could've just been granted a gift from god, to awaken as "the saintess" or something, and go from there. The idea that the 10 years old MC is actually some jaded Jap office drone, is just boring and iiky.
P.S: this also comes across as: "what would happen if Harry Potter frantically focused on making Harry's adoptive family suffer as much as possible?" :v