Asus L402WA-EH21 14" laptop has an unused hidden 2.5" hard drive bay as drive 0 while its 32GB eMMC is drive 1. After inserting a 2.5" drive to the laptop, it would add a D drive to Windows. While it is possible to install new apps and store new file to D drive automatically, users' program data and cache still default to C drive and leads to ever decreasing space in C drive. It seems the only way further reduce C drive usage is by hacking registry keys.
As such, reinstall Windows 10 on the added hard drive seems like a better and more stable solution to address the space issue.
Windows license key is stored in BIOS so reinstalling Windows 10 is a relatively straight-forward process after creating the USB media through Microsoft download site.
After this, existing 32GB eMMC drive becomes D drive and just ignore it afterward.