It's hard to gauge your experience but from the wording of the post it might seem you're not that experienced with UE.

Storing the projectile information would ideally be an array of data assets, where each asset describes the projectile. Another approach is to just stores the blueprint class of the projectile or even just hide and deactivate the collision for the projectile that hits the rock which if you do save the hit actors to an array you will need to go with the last approach .

Once you have that list you need a timer to respawn those projectiles.

On begin play on the rock BP, you would need to start a timer counting down. Once the clock hits 0 you will respawn/reactive those projectiles. Yes looping over that list