Okay, so I just went through a massive binge-read of Mushoku Tensei fics focusing on Rudeus and Roxy, and my search history is basically a map of the fanfiction landscape. The main hub is obviously Archive of Our Own—there's a dedicated tag for 'Rudeus Greyrat/Roxy Migurdia' that's overflowing with content. The filtering system there makes it simple to sort by kudos or date updated, which is how I found most of the top-tier stuff. Wattpad has a surprising amount too, though the quality can be a real mixed bag; you'll find some hidden gems between all the meme fics and one-shots written in texting format.
FF.net is still hanging in there with a decent selection, especially for longer, more classic adventure-style fics, but it feels less active than the other two. A lot of authors seem to have migrated. For really niche or spicy interpretations, I've stumbled upon some on smaller, forum-based sites or personal blogs linked from Tumblr tags, but those are a proper scavenger hunt. Honestly, my weekend disappeared into a 'Rudy learns magic from Roxy but with more pining' rabbit hole on AO3, and I have no regrets.