I have had the same concern as you, and I resolved the problem like this :
I start the breathwork session by using the yoga exercice on the watch and I made only one move. After the session, I go back to the app and select the session. I click on the 3 bottom at the top-right on the screen and select Edit activity : I change the field name by meditation (in my case) and Activity Type by breathwork. I take care about the field name because I have for my own purpose defined differents names to know exactly which kind of breathwork I do (I have around 5 types of breathwork regarding what I would like to increase like energy, relaxation ect...).
By using this way I am able to see after the change on the app somes value as : The level stress before and after, and how many breathes I have done as minimum, maximum and average, and as well the heart rate. There are a good metrics to know if the breathwork technics we applied are benefits ;-)
