Why would you want to need to do all those things manually?
Other than using Calibre the only suggestion I can make would be to use something like MobiMetadataEditor to read existing ASIN (and then use it in thumnail file's filename) or insert (as EXTH 113) if info is missing.
https://www.mobileread.com/forums/sh...98&postcount=1
Also, just to check: are you aware of an issues where Kindle is overwriting cover thumbnails for sideloaded books/docs when Kindle is connected to WiFi?
https://www.mobileread.com/forums/sh...d.php?t=329945
That is also a thing that Calibre fixes (on reconnection via USB cable with Calibre opened).