i don't understand your issue with the jpIV. it's good when it comes out and fucking amazing with vet. it should never die because it's tough, hard to hit, and OKW support units to protect it from other vehicles in most of their main builds.
the raketen has STANDARD penetration for an AT gun. STANDARD PENETRATION FOR AN AT GUN. it's not low, it's not high, it's standard. the zis has the same pen and the pak has an insignificant 10 more at far/medium/long. let's not talk about the 57mm. the 10 range difference on the raketen affects pen insignificantly and is a different "issue".
STANDARD PENETRATION!
Why is why I compared it to the ZiS gun and pointed out you don't want to rely on ZiS guns to kill a King Tiger.
The issue with the Rackten in facing the IS2 the most however is the fact it's really easy to 1 shot due to the high AoE of the IS2 and the extremely close grouping of the crew.
The JPIV's issue is mostly just pathing and being overwhelmed, it has really, really bad turning speed meaning that you need to micro the fuck out of it to keep it in the action.