btw wth you finished chapter 1? there is chapter 2... you are nowhere!
After Mission 31 there are still many missions left that are NOT repeated missions, so forget everything everyone told you there is still a lot of story left. Instal the game and keep on playing you have litterally not seen anything yet!!
After Sahelantropus, and thus the end of Chapter 1, there are missions up to Mission 50.
Out of those 19 missions in Chapter 2, 12 are repeated missions. Then you have Cursed Legacy, which is just about extracting two containers through fulton, Extraordinary which I think is picking up the film canister, and Proxy Wars, which is about to destroy tanks (something you already do in a similar mission from Chapter 1).
It basically means 15 missions play either as a replay of a previous mission in a higher difficulty, or basically demand the player to do very simple mundane tasks.
It's true that he's missing a chunk of story by not doing Chapter 2 - one of those four remaining missions, Shining Lights, is pretty great and pretty much enjoyable in many ways. The one of Quiet is also important, but since the OP is getting bored of the repetitive missions, and he has to endure basically 15 of those, I don't think he'll be able to push himself through them.