First of all, I don't know why the Heter of an incomplete figure mentioned in Seif 7 wouldn't apply to the sun as well. Especially considering that the Shach §25 at the end quotes this Heter as applying to "all forbidden forms" when discussing the sun.
I don't see any problem relying on the Heter of Rabbim, as today in the Western world there is no risk a picture of the sun will be interpreted as Avoda Zara and there is no Chashad. Nevertheless, the Shach §27 quotes Rabbeinu Yerucham that it is Mechuar.
Additionally, the Taz §13 towards the end suggest that a flat picture should be more lenient than even Shakua. Though admittedly the Shach §25 towards the beginning disagrees and equates a flat picture with Shakua which is Assur regarding the sun.