Search Crawler filling up drive, continuous crawls do nothing

Our Central Admin server (also the crawler) is limited to 60 GB on the system drive (where SharePoint is installed).

Initially, a full crawl ran without issue and took minutes. We were happy with that and set it to continuous crawl, which seemed to work initially, picking up changes.

We migrated over about 30 GB of data and documents and continuous crawl couldn't keep up -- nothing new was added to the index for a few days.

I disabled the crawler, then started a full crawl and in an hour the hard drive filled up.

I moved the index, ULS, and usage logs to another disk drive, but this didn't have any effect on the gthrsvc folder on the c: drive.

Is there any way to move the gthrsvc folder? I've tried symlinks but SharePoint somehow figures that out, deletes the shortcut and recreates the folder. They removed the .TempPath property from the SearchServiceApplication object in 2013 and changing the registry keys does nothing -- they revert back almost immediately.