Interestingly, Spock basically never voiced concerns about interference
Well, not exactly never. In "The Apple", we have this exchange between Spock and Kirk:
SPOCK: I am concerned, Captain. This may not be an ideal society, but it is a viable one.
KIRK: Bones was right. These people aren't living, they're existing. They don't create, they don't produce, they don't even think. They exist to service a machine.
SPOCK: If we do what it seems we must, in my opinion it will be in direct violation of the non-interference directive.
But, since Vaal was deliberately trying to destroy Enterprise
, their choices were destroy Vaal or let themselves be killed.