it really depends on where you hang out. The Japanese side of things is huge on Pixiv, obviously—that's where a lot of the original doujinshi and prose stories pop up first. You gotta filter by the レン×ネル tag, and some of the longer narratives there are surprisingly deep, exploring their dynamic as Vocaloids beyond just the fan service.
For English readers, Archive of Our Own has a dedicated tag, and the quality tends to be higher because writers put more effort into characterization. I've found some fantastic slow-burn AUs there that treat them like real people. DeviantArt is more of a mixed bag; you'll find a lot of drabbles and headcanon posts, but the tagging system is a mess. Honestly, unless you're fluent in Japanese, AO3 is your most consistent source for substantial reads. The community there just cares more about craft.
Tumblr still has a pulse for this ship, but it's mostly reblogs, moodboards, and micro-fics. You won't find many multi-chapter epics, but for quick hits of inspiration, it's not bad. I just wish more writers would cross-post from Pixiv to AO3 with translations; the language barrier is the real bottleneck for this fandom.