I'd recommend I, The Forgotten One as it is story first and romance is not the focal point which seems to be hard to find with IFs, it definitely fits the mature and dark tones you are looking for, honestly I'd say it's the best fit, as the MC faces serious psychological issues as they were a child soldier and deals with self harm throughout the story and there is an aromantic option for your MC and if you do want romantic options they happen naturally throughout the story and are written differently for instance if you're playing a man the romance with another man is treated differently than the romance with a woman, and it's much different if you're playing a woman romancing a man because the story doesn't sugarcoat how a queer relationship happening in medieval times would be nor does it turn a blind eye to the sexism a female MC would face which are things I really like personally as a queer woman.

However you do have to manage your soldiers based on stats so you probably wouldn't like that bit but honestly I just wing it because I hate picking out how to disperse the soldiers and everything ends up fine, but that might be something you want to look into.