I think you need to solve this with a sequential convoy. The only issue is if you receive the stop file first, we still need to find a way to pick up the remaining files before the orchestration continues. I have a couple of additional questions…
Do you know how many files will be in any given group, based on something in the file or the file name, or is it unknown?
Will these files arrive in the directory at the same time, or is there a delay in between their arrival?