Another possible option. The University of Colorado has a number of Canvas Enhancements using TamperMonkey scripts. Scroll to SpeedGrader Word Count and follow the instructions to install TamperMonkey and the script.
Also, Data Basic.io Word Counter is developed by educators for educators. The site databasic.io has other text tools such as SameDiff and ConnectTheDots. See the Ensuring Your Privacy statement. There does not appear to have an extension so copying and pasting would be required.