In his book فتح المنان في نسخ القرآن, Ali Hasan Al-Areedh quotes two classic scholars thoughts on abrogation:
Abul-Qaasim Ibn Salaama wrote:
Abrogation in the Quran is evidence of the oneness of God, who said in 7:54, "To Him is creation and command." It is narrated by Ibn Abbaas that he climbed the hill of Marwa and recited that verse then said, "Whoever claims a third power, let him rise. Creation is everything that He created and command is everything He decreed. There are no two words that define dominion like those two."
That confirms God's power. It doesn't prove the abrogation doctrine, nor prove that the abrogation doctrine proves God's oneness. The argument is not that God cannot abrogate; it's that He did and left the abrogated verses in the Quran.
Ibn Hazm Al-Andalusi wrote:
God willed that He forbid things for a while then allow them. There is no particular reason for that, just like there is no particular reason for sending Muhammad (PBUH) at the time He sent him and not before, or like there is no particular reason for the prayers to be five and not three or seven.
Again, doesn't address the abrogation doctrine, but rather God's free will. Rejecting the abrogation doctrine is not a rejection of God's power or free will; it's rejection of a concocted doctrine that cannot be proven.
As for the notion that God's timing and amount of a decree, they are both precise! That is the essence of the belief in Al-Qadhaa wal-Qadar (Decree and Precision)! God says,
So the timing of the revelation and the amount of worship are both precise.