I don’t see how the beam could enter hyperspace partway through. The speed of the beam should come from how fast it is pushed out from the weapon, unlike a ship which propels itself forward and can enter hyperspace using its hyperdrive.
I agree, it is a stretch. A big one. But I don’t see any other way to reconcile two things: 1, the plasma beam being fired from SKB and hitting the targeted planets visibly and clearly at a sublight speed and 2, it travelling across the galaxy almost instantaneously. In my head, SBK has some sort of tunelling device that creates a hyperspace wormhole, the “entrance” of which is near the origin and the “exit” near the destination.