I found this site, which seems to host decent length, and relatively difficult, pieces posted a few times a month.
Here’s the latest one, called いちばん好きな国
The title makes it sound like it might be an N5 level thing, haha, but trust me that the kanji at the very least is high level.
My hope is to find more things like the longer pieces seen on the N1. Blogs and stuff are fun, but they aren’t really the style of writing you see on the JLPT.
Obviously books tend to cost money.
Any other sites you found?