@lumpynose -- perhaps a good solution would be to use Kindle Previewer or Kindlegen to create the mobi file from your epub. Then use KindleUnpack to split that mobi into the "old" mobi /KF7 and "new" mobi/KF8 and "source" files. Then you could upload only the version you want, which would help avoid the bloated file-size.
KindleUnpack is available as standalone, and as a plug-in for Calibre. See the MR wiki
https://wiki.mobileread.com/wiki/KindleUnpack