Would it be worth adding this style tweak? :
Disable ligatures (in kerning mode 'best') * { font-variant: no-common-ligatures; }

(I had a moment when I found ligatures ugly - it has passed :) - and tried this, and it works.)

If yes, where:

Would a sub-menu Hyphenation and ligatures> be ok, to not make this one taller?

(And the more we add features that work only well in "best" kerning mode, the hacky "good" mode feels a bit less needed - vs "fast" that might be good enough for most people that need speed. And when adding features to crengine, I always have a moment I have to think about that mode, and wonder how to make that thing work - often ending not having it work in that mode (font-variants, HB font languages...), and often not testing much it :) So, I don't know, may be we could remove it ?)

I use “good” most of the time due to speed/quality balance. “Fast” is really fast, but some parts of text look a little out of place. “Best” is some times very slow for large books (1000+ CR pages)

OK, so let's keep "good" :)
(Could that be specific to cyrillic ? I have never really noticed any issue with "fast" with latin letters text.)

IIRC, "fast" only does legacy kerning (i.e, kern tables vs. GPOS tables), which might indeed be an issue with some fonts ;).

Unfortunately, I can't remember or reproduce exact reason I didn't like fast so much.
One thing for sure if that if has different spaces between small words, but this effect is insignificant (e.g. produces 370 CR pages instead of 368 in good rendering):
fast:

good:

I am sure about Cyrillic-specific deterioration neither. good's description explicitly warns about lack of support for non-western characters anyway :D

This is on KOReader Stable v2020.03.2 on Kindle PW3 (5.9.7)