Things start to feel more intriguing around episodes 8 or 9, and by episode 12/13 the tone shifts dramatically.

Yeah, it’s a real slowburn, so I understand why you gave up on it the 1st time.

Yes - it’s “the dark magical girl show”, but it doesn’t even feel as dark as I was personally bracing for.

I think there are two reasons for that:

1. You went into it knowing it was going to be dark. At the time, no one knew until the famous Mamiko scene. Everyone thought it was going to be a standard magical girl anime. It was a proper twist.
2. Edgy/dark magical girl shows have kinda become normal and yeah, have really pushed the edge further. Madoka popularized dark magical girl shows.