Finding missing numbers in a sequentially ordered list

Summary:

We have a situation where we setup a piece of logic to auto calculate and store our sub pod names in sequential order but are running into issues with naming when sub pods are deleted out. Looking for a way to have the logic call out the missing number in the sequence and use that instead of incrementing to the next value. Also, open to any ideas on changing the logic behind how I'm auto incrementing the values.

Example: Our situation in the SS below shows we have deleted sub pod PD030_2 from our structure. What this will now do is throw off the logic I created to calc and store the sub pod names, since it relies on the # of siblings to determine the new sub pod value. The next name that will generate when I try to add a new sub pod is PD030_12, which is already a sub pod in the roll up.