D&D 5E Surprise; the kicking down the door type (Starter Box)

[Branduil]

I don't think the rest of the party needs to use readied actions. If Brok the Barbarian has readied an action to break down the door, then Brok breaking down the door is effectively the first initiative of the surprise round. At that point all other characters in the party can act according to their rolled initiative.