The shadow in 3drad is very limited. Do anyone has idea how to create soft shadows using shader?

there was an attempt at this back in the day, unfortunately it was also limited - it seemed to project the castor's textures on the shadowed surfaces from memory. i think it actually used some kind of reflections.

alternatively, you could try reshade SSAO shaders for soft shadows in corners, etc, but it is prone to glitchy artifacts depending on the scene.