But upon an unthreatening, primitive world like Eminiar? On behalf of a few of your citizens?
This aspect could be easily sidestepped: GO24 is for real, but its use in the episode was a bluff. That Kirk intended a bluff was implicit in the fact that the target was the "soft" Eminiar, and not a horrible monster culture preparing to devour the entire UFP in one gulp within the next minute.
OTOH, the idea that GO24 is nothing but a prearranged bluff is certainly an intriguing one. But since this is scifi, we should pay attention to our OTTH as well: in some situations, "national security" (rather than just the hides of Kirk and his crew) could be at stake in a way that would call for the immediate sterilization of a planet or neutralization of its military, industrial or other technological capabilities, and might call for verbal shorthand so that just the three words "General Order 24" would launch the crew to timely action.
But if we look closer at this OTTH, your objection about the level of destruction arises - although I'd apply it in reverse. When Kirk gave the order, Scotty decided to target the planet's "cities and installations". That's an undersized measure if the planet poses an immediate threat to the UFP. A culture so advanced that it could pose such a threat would not be dependent on "cities and installations" to carry out the carnage: stopping there might be worse than useless.
Of course, Scotty would realize that Eminiar was no threat, so regardless of whether he was bluffing, he'd only do as much targeting as was needed for terminating a culture of Eminiar's relatively pedestrian level. But that means that we'd not be seeing GO24 as such in action, just an interpretation of it being implemented.
Finally, in any event, a threat one doesn't intend to carry out only works until someone defies it.
Which reminds us of Garak's advice on never telling the same lie twice. GO24 need not be an order to bluff with the destruction of a planet's cities and installations. It may be a truly general order to apply bluff for blackmailing purposes, allowing the officer in charge to improvise. If so, the enemy could never call the bluff simply by citing precedent. But it would mean that Kirk and Scotty would need to have the same idea about effective bluff, so that they wouldn't contradict each other despite lacking the means to synch their stories.
That's not all that difficult to believe, though. As said,
Scotty had to overwhelm Anon right from the start.
Not too many ways to do that under the circumstances.
Timo Saloniemi