The Master List of Book Clubs labels the Re:Zero and DanMachi book clubs as “advanced”.
But it’s not only the difficulty of the material as such, but also the reading pace which determines the difficulty. So for “not technically hard” books we will set up a schedule that goes much faster than the standard 15 pages per week that’s often applied for Intermediate book club books.
The nice side-effect of this faster pace is that we can read much thicker books; the Intermediate book club limits its proposals to something around 300 pages to avoid endlessly chewing on the same book.