[Quoth]

I use Older Kindle all the time with Gutenberg. I convert to epub2 with smarten punctuation, Justify (doesn't break headings), remove paragraph spacing and set 1.4em 1st line indent.
Under styles remove line-height, white-space
Sometimes I remove extra nested blockquotes in HTML. If editing I'd remove the licence files at the end and all the end-of-file mobi page break divs in one go with the plug-in that does that.

I convert the epub2 to azw3 for Kindle.

I've not tried that file yet. No time.