I started with the anime but that's just because I was unaware of the light novel at the time.
I've trained myself with various other stories to not compare the book version to the movie version or the TV version, and I'd like to think I was able to apply the same dissonance when I picked up the light novel.
With that in mind, my first exposure to the LN was a MT, so that probably also affected me, as I got the plot-important stuff but smaller character details were lost.
When I started with a proper translation, though, I immediately picked up on Myne being a bit of a brat. But instead of turning me away from the books, rather, it cemented the idea that Myne was not, in fact, a reincarny but instead simply had access to otherworldly knowledge. The idea that I was reading through the eyes of a really smart and knowledgeable toddler fixed the "brat" idea, and also helped me pick up on smaller details that Myne points out but that might not in fact be accurate, especially when we got to contrast some of her story with others' in the POV chapters. I really appreciated the author's use of these "blink and you'll miss it" inconsistencies or oddities that helped to portray Myne as an unreliable narrator.
In my opinion, the best way to onboard new readers is to remove references to the Isekai genre, or perhaps to call it Isekai-Adjacent. I have no wish to get into whether or not this is a story of a reincarny; the author said no, and that's how she wrote the character and story. Perhaps any one person's idea of reincarnation is, in fact, how it works in Bookworm, just not with Myne specifically. I think interpreting her as a genius child with special knowledge of how much better things could be alleviates the bitter taste that would otherwise be on the tongues of some of those who understand her to be an adult.
In addition, it might be helpful to come up with our own teaser blurb that hangs a lamp on Myne's behavior.