Hmm, I'm not that big in IOS, but Ibooks/Apple books should offer light novels.
DRM is only important if you are afraid that the books you buy will be removed from the store you buy from, or are afraid that the store itself will close. In this case easy to remove DRM/no DRM would allow you to download the books and keep them to read even after the book is removed from the service.
I only have personal experience with Google play books and Kobo, which both offer the same easy to remove DRM.
The series is available on almost every major online book retailer (Apple books, Nook, Kindle, etc.)