Is it the husband's right to marry another woman merely for the sake of experience, and not due to any shortcomings on the part of his wife who has endured much and changed for his sake?

It is the right of the husband to marry another if he is able to be just among his wives, and there are no other conditions for the permissibility of polygyny. The husband marrying another is not an injustice to the first wife, for the Prophet, peace and blessings be upon him, married while having wives and was the best of people to his family, and he married while having Aisha, may Allah be pleased with her, despite her being the most beloved of people to him. The wife is advised to treat her husband kindly and cooperate with him in obedience to Allah, and the husband is advised to treat his wife kindly and not marry another if there is no significant need for it.