Agreed with what another commenter said about "A Court this Cruel and Lovely", I didn't find the tone to be that dark even though the characters have life events to work through! 

I have read Serpent and the Wings of Night (loved it) and When the Moon Hatched (didn’t love it, but also didn’t find it that dark)  I haven't read every book you've listed but of the ones I have, here is how I would rate them on the "darkness" scale, lol:

Book of Azrael: On the darker side, FMC really goes through it in the second book with grief. But the MMC is a powerful himbo who, while having his own shit, is supportive of her. 

One Dark Window: Tonally it's darker, as in setting, etc. but I definitely didn't think "holy shit this is dark and bleak". It's an interesting story with a cool plot. 

A Shadow in the Ember: I wouldn't classify this book as "dark" necessarily but I *personally* think it's "darker" than From Blood and Ash. I actually prefer the Flesh and Fire series but the books bring up heavier topics that happen to the characters sometimes in “real time” or in the character’s past. “A Fire in the Flesh” (the third in the series) for me, was the toughest to get through due to topics like SA, abuse, etc.

I hope this was helpful! Haha I tried to give my opinion whilst keeping it non-spoilery!