not sure "historical fiction" is a genre, but imo you're on the right track, yeh
theres about five diff genres used in the Bible, can't find the list now but its in here somewhere
[biblicalhistoricalcontext.com]
prolly other places too

Two possible explanations:

1. they actually wanted 36, but that was taken by the Muslims

2. in the Jewish language, 40=bullshit

Generally two people can count that on all of their fingers and toes.

It's fiction

The whole concept of god is a fiction of man’s, the bible is a human construct, therefore everything in it is also manufactured by man. Whether or not 40 is overused in the bible is completely immaterial and may or may not be construed as laziness, but only those who believe that the bible is god’s word need to worry about that, not those of us who don’t. We needn’t give it a second thought.

I don't understand the concept of God being obsessed with a number (except for 42, or course). The Jewish numerology tradition, known as gematria, puts a great value on certain numbers, so if you look, you can find them anywhere. 40 indicates transformation or purification. Raining for 40 days and 40 nights, fasting for 40 days.

I thought the magic number was 45 and that’s how evangelists wiggled trump into their theology?

They hadn't invented umpteen.

Because OBVIOUSLY the Xtian God can't count to 42 which is the answer to life, the universe and everything else.

If you want to believe it.

LOL, possibly by the time Imaginary, Invisible Sky Daddy got around to getting some weak minded human to write the Buybull for him he had already developed Dementia hence when counting using the ONLY things available to him/her/it, the fingers and toes, Sky Daddy just repeated counting them twice until an Imaginary Sky Angel stopped him. LOL

All religious text are made up by humans to control humans.

It is a pun
ארבעים is hebrew for 40 and is pronounced Albi'im
The phrase על באים pronounced Al' byim
is Hebrew for on coming, in the sense of "that which is will be" the on coming storm etc
Therefore 40 days and 40 nights is the inevitable future, the time of inevitable change.

Who knows?
Who cares?

It doesn't rise to the level of "historical fiction".