Sun in Pisces influence a person to be idealistic, sensitive, caring, empathic and peace-loving. Sun and Mercury both makes one prone to being shy, indecisive, daydreaming and susceptible to other peoples' opinions, where the Pisces person can be dominated somehow. People with stronger opinions and convictions can make someone with Sun and Mercury in Pisces insecure about their own beliefs.
Negative vibe people should be avoided as these can leave unpleasant impacts upon Pisces people, as they are very sensitive. Pisces is a weak sign, so people with Pisces should guard themselves very much (as in AoP).
Sun in Pisces bestows psychic gifts along with Mercury in Pisces. There will be a very strong, if not powerful creative ability along with visualization and imagination skills. There is indeed artistical talent. Thoughts, ideas and opinions are often based upon feelings, intuition and hunches.
There is a gift for meditation and the spiritual. Sex magick, and magick dealing with the non-material will come natural for these people. Thoughtforms, elementals, chakra work, sensing energies, energies of all non-material kinds.
Sun in the 7th can attract a partner as well as other people that is very dominating somehow, but the 7th house Sun can also cause someone to be very strong and have strong influence upon others. With Pisces here, this influence is likely exercised through emotions, feelings and convictions, as Pisces tends to project.
7th house Sun people do not like being alone, there will be a strong need for other people and a marriage partner. Though the Pisces sign influences periods of introvertedness. There will be times when the Sun and Mercury in Pisces people need to be alone in order to stimulate their creative imaginative minds.
The above is just a classical interpretation. All of the other planets, houses and degrees need to be blended and interpreted accordingly. There can be supporting and/or offsetting signs. Here is the website for astrology, you can read more about it here:
https://www.joyofsatan.org/www.angelfire.com/empire/serpentis666/Planets666.html
https://www.joyofsatan.org/www.angelfire.com/empire/serpentis666/Astro_index.html