Maybe the fact that you were joining those blocks with bias edges was what made you anxious? I always do HST the old fashioned way.....squares cut diagonally or strips of fabrice cut into the triangle then sewn together or if I am doing a lot of them triangles on a roll or tangles works for me. As far as that bulky center, that can be "twirled" into a tiny pinwheel to eliminate that volcano seam.