How can I completely delete mails from a label with other labels inside?
Labels in Gmail are different from folders. So if you delete a label the selected mails are not deleted recursively like in folders. It’s possible to delete numerous labels at the same time by using Gmail search bar. By typing in label:test1 OR label:test1-test2 it will show you both labels (test1 and test2) and then you can delete them.
To find more information about G Suite in general click here.