What's the best way to identify first sales order for an item and updating that item

A UE (aftersubmit) script, a query that if the item only has 1 sales order, update it

I think you could achieve this with a scheduled workflow. Search criteria to find the "date" of the oldest sales order transaction, run the schedule on the item record. You could use this method to backdate any existing items and filter the searcht o only show items where the field is empty (so once it updates once it will drop off)