Save/Load Scanned Tree
I believe windirstat was inspired by qdirstat, which has a flat text file system for this so this would be a good way to keep them in step.
Recommend taking at look at https://github.com/shundhammer/qdirstat/tree/master/scripts, specifically qdirstat-cache-writer and:
Other options, checkout:
WinDirStat
I often find I run directory scans for others and request the ability to save a directory scan to a file, perhaps XML or JSON, and have the ability to reload a previous scan.