Should I thank my husband for maintenance, when it is his duty anyway?

Something obligatory still deserves appreciation, because thanks nurtures love; it does not cancel the obligation. The Prophet ﷺ said: “Whoever does not thank people has not thanked Allah” (Sunan Abu Dawud 4811). A husband whose efforts are appreciated is keener to give, not just out of duty. Start by saying thank you specifically, for example after he pays for something important for the family. Likewise, a husband should appreciate his wife's unpaid work at home. A household where both are grateful rarely keeps score. See rights of husband and wife.