To get caught up on comics, you might want to check out YouTube channels that do detailed comic reviews or explanations. Another option is to join comic forums where fans discuss and summarize the key points of each issue.
Black Clover isn't caught up to the manga. The manga continues to unfold new plot points and character arcs that haven't been shown in the anime. But both have their own charm and appeal to fans.
I'm not aware of any legitimate fanfic with such a plot. Fanfic writers usually try to stay within the boundaries of good taste and respect for the characters, and this kind of content goes against those principles.
Re:Zero's anime hasn't reached the same point as the manga. The manga has more depth and detail that hasn't been translated into the anime version yet. However, both have their own merits and appeal to different audiences.
The Black Clover anime hasn't caught up to the manga. The manga has more storylines and developments that haven't been covered in the anime yet. But the anime does a decent job of adapting the key elements.