The Kindle Colorsoft has been reported to not expose the system/thumbnails folder for covers, so here's a workaround and pre-built exe.
https://github.com/ciromattia/kcc/pull/767
It just replaces EBOK tag with PDOC, so Kindle reads the cover directly from the file. Please test and tell me if it has issues.
Also, should I remove the ASIN tag?
And does anyone remember when pdoc covers started being read by Kindles?