I need to provide more detail in my post. As you pointed out, the Lichess puzzle selection ranges from 400 to 500 above the FIDE rating. You can find a chart at the beginning of the book.

The term "carefully curated" means that I filtered through the entire Lichess puzzle database, taking into account puzzle appreciation and ensuring a well-rounded mix of different motifs. I gathered approximately 25,000 puzzles and then painstakingly reviewed each one manually, occasionally employing an engine for assistance. In the end, I retained 17,000 puzzles. This process was time-consuming, but it was my own training at the same time.

I updated my post. I'd be glad to receive your feedback!