@eefara mentioned it may help to have benchmark books. I’ve only read around 10 or so books in French, but I feel like La Très Catastrophique Visite du Zoo | L30?? is pretty solidly B1. The grammar forms were all things covered either through my A2 studies or during my B1 grammar study* and the vocab doesn’t go too crazy because the setting is pretty everyday (school, home, zoo).
* I think thanks to the book being entirely narrated in the first person
