Ah. The emphasis on "Believable". Now we are just discussing writing theory.
Front load it.
What I mean is, put everything you think the reader will have problems believing in the first chapter. My cat will not get off my head so forgive spelling mistakes. Anyways. Whatever it is you think people will not believe, you put it in the first chapter and people will accept it. After that, you just need to make sure everything is a logical continuation of whatever impossible bullshit you put in the first chapter.
For example, I have a book on reverse sexual morality. It had some limits on what I could do, so I made, Reverse Morality with Superheros.
In the first chapter of IWS, I made it clear, reverse sexual morality and SUPERHEROES. In the first chapter of FTS, I actually had the MC Isekai'd and implied that the sexual morality of the world was flipped. THAT WAS IT.
So, FTS is much more subtle and I have my hands tied on how extreme I can do shit. The more extreme FTS gets, the less believable the story. But since I front loaded all my impossible shit in the first chapter, and everything since then is a logical extension of the BS in the first chapter, it is "Believable".
In IWS, The MC is about to go on a date with a Plant woman who shrinks down to a flat chested Loli when she is dehydrated and is stacked if she gets enough fluids. How "believable" is that? In FTS, if I had that happen in chapter 23, the reader would be going, "What the fuck?" but in IWS, not a problem.
You want a belivable woman? Establish the traits of said woman and the society she experiences in chapter one, then just follow the logical progression of said established facts.
It is called FICTION for a reason. Just follow the acceptable tropes, and the audience won't have any problems.