I will try to keep this short but as it stands there is no reason to have a melee attack on a bow. We have sidearms. If melee is that imperative for the situation then players can just swap to their sidearm and stab the enemy. There is no purpose, it doesn't do anything to help the bow, it literally adds nothing.
Now, Avex is an Ode bow, the Ode archers have that explosive shot. Just give Avex that for its heavy if we want it to be unique because at least that does something useful.
If the bow NEEDS to keep melee then here are a couple of melee options:
Option 1: the arrow stab puts a bleed DoT on the target. Bleeding targets take more damage from your shots.
Option 2: the bow itself in swing in a wide arc knocking back and slowing all targets hit.
Inb4 "Oh but it is unique!"
Unique doesn't mean good, it doesn't mean helpful, it doesn't mean it has a place.
We can have unique and good at the same time, it doesn't need to be one or the other. We need to remember that sacrificing Arrow Hail needs to have a trade-off that adds something to the bow overall, not takes away from it.
So yes, Avex is a cool looking bow, needs a hefty split of virtues to use, and its heavy attack is utter trash. Please replace the heavy with something that is functional and useful.
Edited to add melee centric options.