Some fanfics also focus on the psychological aspect of their relationship. Reiner and Bertholdt both have complex personalities and backstories. Fanfics might explore how they understand and support each other on a psychological level, perhaps dealing with their inner demons together.
Fanfiction.net is also a great place to look. There are many 'Attack on Titan' fanfics there. Just type in 'Reiner x Bertholdt' in the search bar and browse through the results. However, the quality may vary, so you might need to read a few to find the really good ones.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
Reiner did die in the manga. His death was a result of a series of events and conflicts that had been building up throughout the storyline. It had a major impact on the plot and the other characters.