I think you have probably found the best way given you are a NICA coach. There is no way to have a NICA ride be anything than unstructured at best staying within zone 2-3. You have to manage the practice which is not the same as working out. It sounds like you are doing it really well in light of that constraint on your schedule.
It is volunteering. You are giving that time away for the good of others. Just accept that and do the best you can with the time you have left. The kids are grateful for it. That mindset has helped me deal with the fact I am not going to train like I would prefer. I do use the practice rides to focus on a skill like cornering or wheel lifts, which is probably less critical in a gravel ride.