...drive up to point blank and aim for 4 seconds
Sturmtiger has 40 range, "point blank" is a tad exaggerated.
Still, the tank undeniably remains a niche unit and shares the same problem the Ostwind or Brummbär have: There is no need for a infantry or support weapon killer if generalist tanks fulfill the same job almost equally effective. I think the aoe damage of allrouder-tanks in combination with the tighter squad AI is way out of control and needs to be addressed first to make room for specialists.
Theoretically you can one-shot medium tanks, which is huge, but then there is the problem with death crits. They simply should not happen, at least with this weapon. Every "normal" tank has the chance to give chase and finish a tank off, but not this one.