[Quoth]

You upload docx or epub2 (Amazon admits that epub2 may work better).
Amazon then convert it to all the formats they use. What someone gets depends on which registered actual Kindle, Fire or App they select.

I had issues with the page breaks at chapters with the docx, but it was fine converted by Calibre to epub2. So Amazon told me to always upload the epub2. Same one as uploaded to Smashwords for Apple and Nook and Kobo and also to Google.

Do select Tablet under page settings in Calibre so that images are not resized, assuming you have done them properly for ebooks.

Also every kind of paragraph or heading should be a named style. Only use inline formatting for a few italic or bold words in a paragraph.
Every heading that's not to be in the system TOC should be body level, including the Title.
The docx or epub should not contain the cover. It's uploaded separately on EVERY platform. True also for paper POD or print runs as the cover is a different size to the internal pages.
No headers or footers on the docx upload or local conversion to epub.

The ONLY use for a local Kindle single Format generation tool is your own previews or giving a copy of your own material to Kindle owning friend.
The only use at all for dual mobi (old mobi with KF8 as in azw in the one file) created by Calibre or any other tool is to upload a Kindle version to Smashwords, or perhaps if you are making it Public Domain, some other site. Only Amazon knows what kind of Kindle someone has.
The Dual Mobi from Calibre seems OK on Kindle Keyboard, DXG, older Kindle Basic, Paperwhite 3 and Paperwhite 4.

I found KindleGen very limiting compared to LO Writer (saving as ODT and an extra copy as docx when finished) or MS Word. Then loading docx into Calibre and making an epub2.