Is discussing how well other husbands treat their wives or their luxurious lifestyles considered backbiting if the aim is to encourage one's own husband to emulate them and improve his treatment?

As long as you do not mention the people you are discussing with your husband in a way that they dislike, then this is not backbiting (gheebah) which is forbidden. The Prophet, peace and blessings be upon him, defined backbiting as: "Your mentioning about your brother that which he dislikes." However, one should be cautious that mentioning such matters might lead to the husband's anger. It is preferable to choose an appropriate and effective approach, such as reminding him of the ethics of the Prophet, peace and blessings be upon him, and the righteous.