3. I draw my little ball. I also do it by expressions to work faster. So I create a user knob named Size in the Radial Properties panel. Then in the area fileds I put these expressions:

root.SunPos.translate.x-size/2
root.SunPos.translate.y-size/2
root.SunPos.translate.x+size/2
root.SunPos.translate.y+size/2

In this way, I draw a perfeccircle with only a slider!!Crop properties.




Now we branch a VolumeRays from the resulting little ball and in its mask we connect the output of the sky.
Here we create the length and color of the sun rays, so the important things are the Ray Length, Initial Volume Color and End Volume Color controls.



I also put an expression in the Volumetrics center fields :

SunPos.translate.x
SunPos.translate.y

You know ... for it to move accordingly with the SunPos coordinates...




Good, now we are going to create the rays and sun shapes. Well do it with a flare and a radial.
So we branch a Flare, a Radial and a Shuffle from the last Crop.
Investigate a little bit in the Flare parameters to get some beautiful rays. The secret is in the Multi Tab / Multiple Flares.
Then with the Radial I draw a little ball to hide the center of the flare and give it a round shape. This node is a copy of the previous Radial node. It also has a slider called size to control the size of the circle. (By expressions I control the Crop and the previous Radial from this nodes slider.)
Then with the shuffle node I copy the alpha channel from one of the RGB channels.