Well, then lets do it in autofire and see how it goes.
and i can tell you right now that it would be obscenely OP. for reference, it would be a slightly less powerful Pak43 that could move.
the simplest thing is to fix the piercing round so that it actually functions.
the problem with the ISU is that, while not as powerful as the elefant for multiple reasons, it's still a very strong vehicle AND it's effective against infantry. for those reasons it's a bad idea to buff the fire rate very much and buffing the accuracy would be really bad (more squad wipes and squad wipes are bad in a game about unit preservation). that makes the ISU tricky to buff as changes have to not make it kill things too much faster or it rapidly becomes OP. making concrete piercing more reliably, and maybe cheaper is a good start and if that's not enough then increasing the pen on the AP round is another good place.
the ISU is a very solid tank on more open maps and supported by an SU-85 or two for spotting it will shut down anything other than a pak43, elefant, or jtiger. it's definitely harder to use than most units though and the commanders have fallen out of meta for a variety of reasons and so the ISU doesn't get much use anymore.