I think they are really trying to not add or subtract greatly from their existing story. The minute they start stretching out the story or padding it with in between years is the moment they will start breaking a somewhat implied promise with the viewers. They’ve been very vocal about the fact they are adapting this specific story. Also the in between years would take on the effect of a prequel where all suspense is taken away since people know everyone must stay alive bc they are in part 2.