The ISU152 does not 1-shot everything. I think you have it confused with the Sturmtiger.
it one shots infantry squads if it lands in the middle of the squad (which it often does), even if youre trying to spread them out using drag+right click
honestly i'd rather see all long range anti-infantry ordnance (mortars, isu, artillery) changed so that their AoE is large, but their damage decreased. a squad as a whole will take a lot of damage, but wont run the risk of being squad wiped. currently there is no 'counter' to a random shell landing on a squad and wiping it. its all RNG based. sure, you can argue 'well dont be in the firing range', but some artillery is long range enough that this is realistically impossible (120mm, isu, infantry support gun but i ahvent seen it wipe squads often).
with a damage decrease and an aoe increase, long range firepower would in essence 'soften up' enemy infantry, to make your own infantry have an easier time attacking.