Is it considered disobedience for children to refuse marriage in order to complete higher education, then insist on performing Hajj with their own money after their father's death, knowing that the father had refused Hajj with the children's money and prioritized marriage?

If Hajj and marriage conflict, marriage takes precedence if the person needs it and fears falling into sin; otherwise, Hajj takes precedence. There is no obligation to postpone Hajj merely for the father's desire. As for prioritizing marriage over completing higher education to please one's parents, it has been reported from Imam Ahmad what implies its obligation in this case. There is no harm for a father to perform Hajj with his son's money, but regarding the obligation of Hajj for someone to whom money is offered, there is a difference of opinion among jurists.

Therefore, you must hasten to perform Hajj unless you fear falling into sin by delaying marriage. And seek Allah Almighty's forgiveness for having initially disagreed with your father regarding marriage.