In the comics, Drax is not dumb. He may not be the most sophisticated character, but he has his own kind of wisdom and skills that help him in battles and adventures.
Overall, Drax is quite strong in the comics. He can go toe-to-toe with many powerful villains. His physical abilities and combat skills give him an edge in battles. Plus, his determination and drive to fight for justice add to his strength.
No, Drax doesn't kill Thanos in the comics. Thanos' fate is usually determined by a more complex set of circumstances and interactions with other characters.
Yes, there are some. You can check out fanfiction websites like Archive of Our Own. There might be various stories exploring the relationship between Drax and Mantis, from adventure - based ones where they team up in different scenarios to more character - study - like fics that delve into their emotional bond.
One common theme is friendship. Given their interactions in the movies, many fanfictions explore how their friendship deepens over time. Another could be adventure, as they are part of the Guardians of the Galaxy and often get into exciting situations together.