Doing open world game without planning out each level on paper (or photoshop or any other software) is a huge pain the neck.
Draw a map, split it into chunks (like Bethesda does) and work on each chunk separately. Also, I’m pretty sure you can send other chunks to the other members of the team.
Like people mentioned earlier you can stream levels. Basically it will be visible from distance, but unplayable. My advice, do something smaller. Do one location, which is surrounded by something, mountains or a barrier. And then you will see how quicly and how complicated things will get.