18-135mm is not a main lens. A main lens has a fixed focal length.
The focal length depends to a large extent on your taste, but in any case, you should buy a lens with a wide aperture (low f-number). This makes it easier to obtain blurred backgrounds.
I loved taking portraits at 200-300 mm (on a trimming sensor like yours) that often resulted in images that the subject had not even noticed, which is the most natural expression you can get.
Now I recommend a 70-200mm / f2.8, which is expensive but you get a good background blur and the quality is really good.
You will need 35 mm or 50 mm if you take a picture of the whole body or group photos and not just the face. In addition, you get more context if you use wider angles, because the viewer will understand what the stage was and not only see a face with a blurred background.
I would not recommend 35mm and 50mm. Buy one of those and use your feet.