[Nyororin]

With the examples you have, the meanings are very different.

I general, すぐ tends to be just "soon", and そろそろ is similar to "pretty soon".
There is a difference in how definite the remaining time is.

In your examples though, you use すぐに and 今すぐ, which are different than a solitary すぐ.

For 1a, the bus is going to be leaving pretty soon.
1b, the bus is about to leave. It will be departing immediately.

For 2a, you will be eating breakfast pretty soon.
2b, you are going t eat it immediately.

In both your examples, すぐ is not soon, but rather immediate.