What is better for performance?

Hi, everyone! At first, sorry about my poor english.

So. For example, I have many rooms in my game. Each room is a prefab. And I need randomly generated level. Let's say, that amount of rooms(prefabs) equals 50. They should be connected by doors. And player always starts in the same room. I plan to carefully optimize the graphics, using modular assets and tiled textures. But, nevertheless, it will be next gen with photoscanned materials.

I have to choose:
1. Generate whole level, while the game is loading

2. Each time we open door in the current room, add new random room from pool of rooms, and attach it to this door. And when we exit that new room, it should be unloaded.

3. Generate 5-10 rooms, when the game is loading. And then everytime do it as in paragraph 2.

4. Other way?

And is it smart to use modular design in such game?

I should say, that I'm not a programmer. I'm beginner game designer. And I need to think through everything and write technical documentation.

So, I hope you can help.Thank you in advance.