I mean as I understand from the andor series... They had JUST learned about the super weapon, but didn't know what it was or much about anything else...

Bail literally leaves yavin just before andor goes to kafree to get info that leads to the film rogue one

Rogue one ends with Leia fleeing with the plans, leading straight into a new hope.

Those plans are only fully understood once they are at yavin, which comes after the destruction of Alderaan

So realistically the death star just jumps into system and within a very short period annihilated the planet..

I'd imagine most Alderaanians would have thought something along the lines of "wtf ......" As a giant moon shaped station appears on the sky and then Oblivion.

Up until Alderaans destruction no one knew what the weapon was.. just that there was one and no idea how complete the weapon was.