That is a good site idea, thanks for sharing. One of the main time wasting problem I had was finding books at an appropriate level (readability) to read. Their book collection on Languageroadmap is small for french but there are some books there that matches my reading list too, good to see that.
Currently I am doing something similar inside Calibre using a plugin called Count Pages, it support adding readability score to each book and sort them accordingly. It can also display and sort by the number of words in a book.
The readability score is probably not accurate since it is probably designed for english books but I find it mostly accurate so far when I try to read the books to confirm the difficulty. I think some of the scores takes into account sentence length as well so maybe that's why it still works for language like french. Usually easy books use mostly short sentences.