To access the new side content you'll have to load a previous save file, yeah.
We know things seem tight time-wise in the game. Some of that is purposeful on our part — there is not going to be enough time to do everything, so players should reflect on what they want to prioritize and what they'd like to see. This means everyone will have their own very unique journey through the game.
At the same time, I'm also aware this issue is exacerbated by the relatively small number of in-game management days. What I can say is that as we add more main story chapters this will start feeling better.