Actual box sets? Or long series? KU sort of doesn't matter. You can read 30 books in the month if you do 1 a day, regardless of being in box sets or not. No box sets to recommend, as the ones I do recommend have never been in box sets. I can recommend some long series though.
My publisher has put my fantasy series, First Civilization's Legacy, in a box set (not LitRPG, however) on sale, if it might be of interest. https://www.amazon.com/gp/product/B07NCQJHMZ/