I'm trying to create an ebook for sale on Amazon. I have Calibre and can sort out an epub version with automatic hyphenation, but when I convert to mobi it won't echo this (and I really want automatic hyphenation because I hate huge spaces). The answer, I'm gathering, is to convert to azw3, which does support automatic hyphenation. The problem I'm having now, though, is that my pc isn't reading the azw3 file and so I can't send and check it on my Amazon Fire. Any suggestions?
If you upload valid epub2/epub3 books to Kindle KDP, Amazon will automatically convert them to KFX books with enhanced typesetting, if the device/app supports enhanced typesetting.
In order to preview enhanced typesetting features, you'll need to install Kindle Previewer 3.
Once you've installed the previewer, you can also install the KFX Output Calibre plugin and generate KFX files that you can sideload to Kindles and Kindle apps.
For more information on enhanced typesetting features also see Appendix A of the Kindle Publishing Guidelines.
Several Kindle devices support enhanced typesetting, so you should expect the body text of ebooks to include automatic hyphenation.
When I format books, I don't do anything special about hyphenation, except that I disable hyphenation for chapter and subchapter titles.
Hyphenation depends more on which device you are using than the ebook itself.
With regard to spacing, I have to wonder where your source material is coming from. Did you create it in MS word, or are you converting it from a MS Word (which throws in a lot of random HTML code and styles)?
With regard to converting from epub to mobi, I would use Kindle previewer instead of calibre. https://www.amazon.com/gp/feature.html?docId=1003018611