I agree Kindle font sizes without the hack are pretty poor. But then I patched Kobo font sizes also, I couldn't find that really satisfying size otherwise.
From my experience with my unpatched Libra I recall huge gaps at the bottom of the page when reading epubs (widows/orphans and also I think there was a bottom padding with epubs). Patching and modifying the css took care of those issues. Plus the lack of bolding options for custom fonts, again fixed with a patch.
As to Kindle line spacing and margins, I'm fine with those. I don't like ultra narrow margins and line-heights.
Of course everyone has their preferences, likes and dislikes. I'm not saying my opinion is some universal truth.
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