Most efficient hard drive workflow for FCPX?

Current workflow:

- Maxed out Mac Mini (late 2012) w/ Fusion Drive
- FCPX app running off of the Fusion Drive
- WD MyBook 4TB HDD (5400 RPM) external hard drive where my raw data and FCPX libraries sit (I have 1 TB left on this drive as I shoot in 4K - getting a MacBook Pro in a year)
- WD MyBook 4TB HDD (5400 RPM) external hard drive that CarbonCopyClones my Mac Mini HD and my other WD MyBook HDD as a means of backing up (100 GB left on this drive)

This current workflow is not ideal. It's slow and editing / rendering is taking me way longer than it should.

Without replacing the Mac Mini (which I'll be doing in a year) what is the most ideal way to speed up this workflow and create a long-term (higher capacity) backup system.