The text of Weird seems to imply that the frightened condition is a prerequisite for the target to take damage. Hero's Feast makes a party immune to the frightened condition. Would this mean that Weird would have no effect, i.e. PCs under Hero's Feast's effect would not take damage?
Yes, Hero’s Feast makes you immune to Weird.
At the end of each of the frightened creature's turns, it must succeed on a Wisdom saving throw or take 4d10 psychic damage.
Since hero’s feast makes you immune to frightened, you cannot be “the frightened creature”, and only “the frightened creature” takes the damage from weird.