How to best track ballet class (mostly barre) with my Garmin Swim 2?

Cardio is just fine. There is no difference in the metrics like Calories, heart rate, etc. anyway, regardless what activity you select, so you can also use practically any other available indoors activity type. No need for creating a new activity type for every of tens of thousands of existing human activities. Calories are calculate from your heart rate, so whatever you select, the result will be the same. Wingsuit flying requires quite specific tracking, but the waste majority of indoor activities fit quite well under cardio, and do not need any specific tracking. If you need something not available on the default Cardio screens, you can customize them, adding the data fields you prefer.

Thank you!