The seige mode has the "benifit" of locking the vehicle down. In a balance perspective that adds counterplay and requires the SOV player to defend the area ESPECIALLY because the su76 is made of tissue paper. Free barrage can be canceled and used on a whim which has a higher chance of abuse en masse
This is my whole argument for a siege mode: You can utilize a playstyle of softening up enemy defenses for assaults, blunting enemy blobs in defense, and be punished if you do not respect your siege mode, just like in the case of the kv-2
Meanwhile fortified armor lets ferdi have over 100 range in a stationary mode, panthers getting TD range, and you can IMMEDIATELY become mobile if you need to. There's no reason why the su-76 cant receive it's own siege mode, it fits the faction, is reasonably balanced, and the unit will have relevance throughout the game.
Honestly if people are afraid of spam, abuse, or whatever, but still gave wehrmacht a hull down with no downsides, idk what to say anymore