Just FYI. We're phasing out SkeletonUtility's SubmeshRenderers soon because it causes some problems with multiple-page atlases, makes unnecessary draw calls, and is a little finicky.
Something more robust is coming soon. Probably next week. You can check this branch on github for progress: https://github.com/EsotericSoftware/spine-runtimes/tree/unity-skeletonrenderer
So you can use that for now, but try not to make something too elaborate.
You will still be using SubmeshSeparators on SkeletonAnimation so feel free to fill that in.
The next update will remove SubmeshRenderer functionality but will preserve the fields so you can still have your old data for a while.