Glad you asked, because I'm leery of automatically using the "opposites" approach when it comes to Dignities. In 12/12, Sun in Aries is a Sense of Purpose placement, because the Sun is Domicile-ruler of Leo, Fixed-Fire, which connects fiery Aries to Fixed-sign Taurus. Leo has the Element of Aries and the Modality of Taurus, making it catalytic in the transition from Aries to Taurus. So, the Sun is the Motivational-ruler of Aries.
Since Sun in Libra has no 12/12 connectivity, the question is, should it be labeled "Purposeless", as the opposite of the "Purposeful" Sun in Aries? Sounds to me more like a vacation from having to pursue a goal, time to enjoy beauty, romance, and luxury. But in our Purposeful state, living as we all are in the Sense of Purpose Age of Capricorn, such things are considered a waste of time, so in that sense it could be considered an "afflicted" placement, even though it has a good result in my opinion.
As for Venus in Libra, happily Domiciled and strong.
Mercury in Libra, innovative thinking, shaking up the conventional wisdom.
Moon in Libra, with the Moon being Regulating-ruler of Libra, Sense of Direction, mind and emotion in harmony.