Hi. I went your path )from Audition to Reaper some years ago and kept using Audition for certain tasks but now find myself using it less and less...
Anyhow if I understand you right, I think you need to use Regions to define the renders. Kenny has a good video on Regions here:
https://www.youtube.com/watch?v=rPUFHabfdI0
And a guide to rendering regions is here:
https://www.youtube.com/watch?v=LcRTWoNT3GY
Is that what you are wanting?