Is it permissible for me to marry a girl my father initially approved of, then rejected without a valid reason, knowing that my paternal and maternal uncles recommend her, and would my marriage to her without my father's consent be considered an act of disobedience?

Obedience to parents is obligatory if they forbid their son from marrying a specific girl for a valid reason, unless the son fears falling into haram due to his attachment to that girl. This is based on the statement of Imam Ahmad: "If a man fears for himself and his parents prevent him from marrying, then they do not have that right."

It is also advised to strive to convince the father and be gentle with him, and to supplicate to Allah to open his heart. However, if the father insists on refusing, then it is advised to leave that girl out of obedience and dutifulness to the parents. For whoever leaves something for the sake of Allah, Allah will compensate him with something better.