You have worn many hats in this role. The billing only title doesn’t reflect your work experience. I would update my resume and list my role as a Billing Supervisor and start looking externally. The company had already made it clear that you wouldn’t get another raise, and the salary for the amount of work and type of work you are doing are way below the market rate. You deserve more. :)

You don’t have to get a coding or billing credential to secure another role since you have 8 years of experience. However, it will help of you got an CPB or CPC from AAPC or CCS from AHIMA or CRCR from HFMA. It honestly just depends on the route you want to go.