One I personally enjoyed and also has some really interesting dark vibes is Kuro. The only caveat is that Kuro doesn’t have full furigana (but is not really that much harder than Happiness)
I haven’t read it but if you want something from the same author that is also at a similar level, there is Chi no Wadachi. From the reviews it seems to be highly praised too.
Slightly less dark (but still dark!), there’s also Wadanohara and the Great Blue Sea which I also liked quite a bit, and has some really good art.
If you not necessarily want similar themes, just something on a similar level, I can recommend Chiisana Mori no Ookami-san which is very lighthearted fun and considered slightly easier than Happiness.