I would argue that rather than buffing the armour, making the brummbär more resistant to all AT, they should increase the barrage range. It is absurd that this vehicle can not fulfil the supposed role of attacking stationary positions as its vet ability has the same range as AT gun. Increasing the range of barrage by 5 and 5 more at vet 3 would up to total + 10range. This would act as a pseudobuff to the durability. This would be more balanced way to make stupa perform better in its supposed role without becoming more resistant to the allied TDs which should hardcounter this designated AI vehicle.
EDIT: I'm also here to protest grave injustice: Hull down does not increase the range of the barrage.
If you want to take a position from afar, build a pzwerfer. If you need shock value then the brumbar is your choice.