What is the halacha regarding painting a full sun? If this is not allowed and I have already painted a painting with a full sun, am I allowed to derive benefit from this painting? Thank you

Shalom, The Shulchan Aruch (Yoreh Deah 141,4) forbids images of the sun or moon both in a raised or sunken form, unless they are for educational purposes. Whether flat drawings are forbidden or not is a point of argument, with some opinions permitting it, whilst others forbid it (see Tzitz Eliezer Volume 9, 44). (Your question indicates that you are aware of the halachic opinions who differentiate between a full image and a partial image, which they permit). Where the picture has obviously no connection to any form of idol worship, and is clearly only decorative, those who are lenient certainly have halachic opinions on whom to rely - and it seems to me that the prevailing custom, both nowadays and throughout history, is to be lenient. On the other hand, there certainly are opinions that are strict (see Igrot Moshe O.H. volume 5, responsa 9, paragraph 6). In relation to deriving benefit, one is allowed to do so. Blessings.