The capacity to choose evil does not ADD anything to God’s power, ergo it cannot be a positive quality to choose evil.
You seemed to imply that the lacking the capacity to choose evil would be a limiter on God’s free will. I am saying it would not be because choosing evil would be a negative attribution and take away or lessen God’s power because evil is always a privation or deprivation of some positive capacity or other. That is simply what evil is.
Choosing evil always detracts or takes away from being, so in the case of God, choosing evil would necessarily make God less than omniscient, omnipotent and omnibenevolent.
Even in human beings, free will is the capacity to choose from an array of goods provided they are all licit goods. By choosing evil, the capacity to choose is marshalled against itself to undermine the very capacity to choose itself – that is what evil is and why it is harmful. It blinds and disables the virtues or powers of the soul.
Why would God choose to act against the power of his own will - again what evil is?