Dragon Ball Super II: Orange Hillbilly Dooms World... Again.

[Douglas]

I'm infering from the fact that it took the Super Dragon Balls for Zamasu to become utterly invulnerble to all forms of harm... Except being Hakai'd or Erased.

There is no proof that Zamasu needed the Super Dragon Balls for that wish, just that he chose to use them for it. He likely didn't even know that the normal Dragon Balls existed - he only knew about the Super ones because he saw them in the background on Gowasu's TV when watching the inter-universe tournament. Also, immortal Zamasu vs Hakai has not been established which would win - the Zamasu that Beerus destroyed had not yet made any wishes.

If not even the Super Dragon Balls can make you completly and utterly immortal so that nothing can kill you, then obviously the significantly weaker normal balls can't either.

The show makes a very explicit point through dialogue that Zamasu's immortality was partially compromised when he fused with Black, because Black was not immortal.