Thanks for the kind words.
There’s no “grammar” category at the moment. Texts in the public library have one of five categories, picked by whoever uploads them: Fiction, Non-Fiction, Song, Dialogue and Other. If you want to keep grammar focused texts together, you can put them on a shelf: go to Library, then My Texts, tick the texts you want, then “Arrange on shelves…” and “Add new shelf…”.
Regarding the levels, @r.twelve is right that it’s a formula rather than AI. It combines the Automated Readability Index (based on average word length and sentence length) with the percentage of words in the text which are among the 2000 most frequent words in that language. To be honest it does have a lot of room for improvement. There’s more detail here: How does Readlang know the difficulty of a text?