The Storm Shadow have very similar capabilities to ATACMs, I don’t think it will massively increase what Ukraine is already capable of doing.
I think the reason the US is dragging their feet to release them is that once Ukraine starts using them, Russia (and their allies, Iran, China etc.) will have enough data to start developing countermeasures against them.
Russia doesn’t need Ukraine on its own, they need Ukraine to get access Poland, Romania and Moldova.
They already switched into war time production, and if they manage to get Ukraine they only need a year or two to replenish all military equipment.
People really underestimate Russia and that’s the worst thing you can do to get caught pants down. If Russia senses there’s a chance to succeed they will move on.
Another thing the West seems to be ignoring is that they think the war is just what is happening in Ukraine, when the majority of their operation is disinformation, subversion, and destabilization.
The popularity of far right politicians (who for some reason are friendly toward Kremlin) or BRExit shows how effective it is.