The decision to select a ‘project-based’ or ‘part-time’ Sourcing Agent will be based on your 12-month launch plan. Personally I have always favoured a retained agent to help with sourcing & quote management, order placement, production order tracking, inspection, freight tracking and reordering and then repeating the whole process for the next product, etc etc. But this will be a decision you will make based on your budget and sales planner.
You can test an agent out, or you can use a recommended agent and just make sure you vet them to them. Sometimes when you don’t commit upfront, you find it that you are both only thinking about a ‘short term transaction relationship’. So it is up to you. You could mention that this is the first time you have worked with an agent but having done it yourself you admire their skill-set and truly value their expertise. And you would like to engage them for one source project 1st with a possible view of extending this after the first project to an ongoing sourcing assignment if they would be interested and available after the first project.
On your ‘side note’ comment, YES, it is great that you can see, first hand, the work and value a sourcing agent can provide. Taking it on yourself does indeed suck your time, add more stress and test you in an area where you are are not the expert or have the expertise.