II testing this two ways - first, defrosting the whole plastic bag in the reusable bag and second, putting the frozen chunks in the bag to finish defrosting.
Whether the first option is feasible depends a bit on the width of your bag. I froze 5 oz of milk in a Lansinoh bag and was able to mostly fit it in a Junobie bag (it was a little long). You can see this above
Again, the goal is to avoid losing milk due to leaking during the defrosting process.