The only way to reduce line spacing for azw3/KF8 books is to edit font files (and change metrics). When you have "fixed" font then you either embed font files in
each azw3 book or if possible on your Kindle model (like via "USE_ALT_FONTS" on Kindle PW1) add you custom font to Aa menu.
Try this
pre-edited Charis SIL font to test and see if you like it and can use it permanently. Just download the files and embed them via Calibre option in Convert Book window.
If you wish to use your favorite font you should ask @JSWolf for instruction on changing metrics values for your font.