Probably one of the easiest ways to excuse students is to click Grades in the course Navigation menu. I would then apply a filter to only see the section of the students you want to excuse from the assignment. Enter "ex" for the grade for the first students in the column for that discussion. Press Enter/Return on the key board and then do the same for each student until all of them have been excused form the assignment.

I tried to set the default grade to Excused for the assignment once I filtered for the section. It gave a message stating that the default grade had been set for the number of students in that section, nothing was changed in the gradebook. I am thinking that maybe you need a numerical grade for this to work. Feel free to try this though to see if it works for you. To set the default grade, hover over the title of the assignment in the Gradebook. Click on the three dots that appear to the right of the title and select Set Default Grade to se if this method works for you. (Only typing "ex" gave me an error. Typing "excused" Gave me a confirmation method, but nothing changed in the Gradebook.)

Good luck and I hope you find a way to make this work with your workflow! Would love to hear your results, which may help other Community members too.