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.
Their relationship would likely deepen. The impending arrival of a baby would bring them closer together on an emotional level. They'd have to communicate more about their hopes and fears for their family. For instance, they might have long talks about what kind of values they want to instill in their child.