How to make projectile pass through block?

You can make your projectile pass through blocks, but I'm not sure if there's a great way to do so based on the block's hardness level. Basically, you need to lock your projectile element, and replace this section:

@Override
    public void tick() {
        super.tick();
        FlameOrbFlyingProcedure.execute(this.level(), this.getX(), this.getY(), this.getZ());
        if (this.inGround)
            this.discard();
    }

With this:

@Override
    public void baseTick() {
        super.baseTick();
        FlameOrbFlyingProcedure.execute(this.level(), this.getX(), this.getY(), this.getZ());
    }
    public void tick() {
      this.noPhysics = true;
      super.tick();
      this.noPhysics = false;
      this.setNoGravity(true);
   }

Where 'FlameOrbFlyingProcedure' is replaced with your projectile's 'on tick' procedure. You can remove the 'nogravity' bit if you want it still fly like a normal projectile.

You may also want to make your projectile a living entity, rather than a projectile element; there's some weird stuff with how projectile elements are rendered when their physics are disabled, and you can make a living entity that behaves like a projectile when it detects another entity nearby.