Is saying "Allah (Alone) is Sufficient for me, and He is the Best Disposer of affairs" against one's father considered an act of disobedience to parents? And would I be sinful if something bad befell the one who wronged me after I sought Allah's sufficiency against him? And is seeking Allah's sufficiency a supplication against the oppressor, or merely a remembrance of Allah?

It is not permissible to supplicate against an unjust parent due to the disobedience (Uquq) involved therein. As for the saying: "Hasbi Allah wa ni'mal Wakeel" (Allah is sufficient for me, and He is the best Disposer of affairs), it is not a supplication against him, but rather a delegation of affairs to Allah the Almighty and seeking refuge in Him, which is permissible and is not considered an act of disobedience. If the unjust person is afflicted or punished after this saying, there is no sin upon you. Ibn Baz advised against supplicating against a parent, and to suffice with saying: "Allahumma ihdihi, Allahumma ikfina sharrahu, Hasbuna Allah wa ni'mal Wakeel" (O Allah, guide him; O Allah, protect us from his evil; Allah is sufficient for us, and He is the best Disposer of affairs).