The dried leaf of Mullein plants has been reported to thin mucus in the respiratory pathway. It is often used to make as a tea. Mullein also comes ground up and in a pill form.
https://www.webmd.com/diet/health-benefits-mull...
The best remedy by far is staying hydrated with water, juices and other fluids and foods. I too agree that any OTC meds with DM should be avoided. Dissolved mucus requires enhanced breathing efforts and sometimes coughing to expell it from the body. DM is a drug that not only suppresses coughing but slows the breathing reflex. DM can also interact with many other common meds.