One way to nerf the ISU could be to lower the frontal armor and give it some more HP instead (obviously not so much that it results in a buff in total).
This would mean that lower tier vehicles would work more reliably against it and give it longer downtime due to repairs. However it would also be able to stay longer in fights and be contradictory to the general design of casemates (strong front, weak rear), since the difference between front and rear armor would be smaller.
To be frank, would it need a health buff to accomidate the armour nerf? I know that's completely contrary to what I have been saying but it already has enough health to be forgiving, dropping say 40 armour, if screened properly it should be fine. With ptrs penals, oorah cons, t34 rams, 30 mu mines and 60 range TDs, getting in there should be rewarded