I met the same problem of not being able to shrink the Windows OS partition C: as much as I plan to due to unmovable files.
Having learned from this post, I would like to "turn System Restore off and remove System Volume Information folders". So I was wondering where and how I can "turn System Restore off and remov System Volume Information folders"?
This is the only way I can find out to solve my problem. If you have other way please let me know.