Is it considered a sin if I manage my car myself, despite my fear of the anger of my parents who are mismanaging its profits through extravagance, while I am keen on honoring them?

The rights of parents are great, and Allah has coupled them with His own Right. It is obligatory upon the son to honor them and to comfort them with his wealth when they are in need. It is permissible for a father to take from his son's wealth, under certain conditions.

If the parents do not need the income from the car, it is permissible for the son to oversee its operation after they have lost its profits, but with a gentle approach, without causing offense, remembering Allah's prohibition against even saying "uff" to them.

It is obligatory to maintain ties with parents, honor them, be good to them, and spend money on them, for the parent is the middle of the gates of Paradise, and whoever reaches his parents and they do not admit him into Paradise has indeed lost.