If your EPUB looks exactly how you want, turn off everything in the conversion settings that messes with the formatting. So, check "Disable font size rescaling", and uncheck "enable heuristic processing", set "Text justification" to "Original", etc.
The CSS I use for my EPUB has justified body text, but when I convert to KFX and sideload to the Kindle, I can choose the ragged-right option and it changes the display as expected.
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