but she doesn't seem to have that age to her that Nine onwards do, that burden of her blood-stained past, and sits much more comfortably alongside the Classic Doctors than the New ones. I presume this is a conscious decision, but I need that link, the acknowledgement of her history and her past to fix her properly in my head.

Yeah. This, I think, is also my biggest issue with her. I wouldn't mind it so much if we'd been given an explanation for it, but as it is, however, she just goes from near-suicidal Twelve to someone who doesn't seem to care about anything.

(the centrist ideology of the series also doesn't help, but fits the characterization so I too assume that it's a conscious choice)