The rebellion network that Lucen built is huge. However, it was quiet and the Empire did not notice.
It incites citizens who rebel against the Empire, and eventually becomes a threat to the Empire.
People support the cause of hope and join hands. Gradually, they build up military power and sporadically deceive the centralized system.
Concentrating military forces is very dangerous. There is no chance of winning if they fight the Empire head-on.
They decentralize resources and deploy military forces. Of course, very few people know the true nature of all the rebellion activities. They are not informed.
Cassian grasps the threat of planet killers. Even if they attack, they cannot deal with it with the scattered military forces that have been there until now.
They need to gather and consolidate some resources and military forces in advance. The problem is where to do it? So it seems they will make it a moon on Yavin 4.
The Empire will notice the movement of these forces.
Tarkin was irritated, "It must be somewhere..."
To make matters worse, the base they set up on Yavin 4 has more resources and personnel than they had expected, making it a de facto powerful base. Because the Network had not allowed the establishment of bases until now, they had not anticipated the physical risks of using it.
Lucen uses the Network to keep the location of the base a secret.
I don't think Lucen would tell the Empire that there is a base on Yavin 4, even if it meant sacrificing himself.
In the end, Leia doesn't tell the Empire either, but she is tracked down by an Imperial beacon installed on the Millennium Falcon.
However, at that point, the plan to attack the Death Star is put into action.