this is a good question man,
I am often in between.
Some suggest to focus on one genre, the one is better for you, the one you love and where you think you can do your best. Also, it could be your “trademark” you “brand”, maybe a couple of genre is fine too.
Others suggest to jump into most different genres you can, to have more chances to get a deal, I guess is not a bad idea but surely every production, in every genre, need to be top notch.
And usually is very difficult to be an expert on every genre (lucky one who can do that!).
I am into rock instrumental, I am exploring more orchestral and hybrid but as a rock guitar player that’s the genre I am working on.
Even once you decided your genre, I think is still not so easy to find the best “structure” for the best library.
There are some golden rules indeed, but even in just one genre there’s tons of different colours to cover.
Just my 2 cents, I would focus on a couple of things you can really do at your best,
best of luck man!
Max