Hikaru has always impressed me on her range tbh, her lower matches Keiko at times and she had to do Kajuriagos that Wakana did too, for being the "newbie" compared to the other two, she got put to use
Oh yes, she was surely put to use. Kajiura's music is not easy to sing!
Hikaru really doesn't naturally have a heavy or weighty voice, but growing up she was required to sing the music Kajiura was creating, which was anisong and thus often epic, dramatic, and fast tempo. As a result of the vocal abuse/strain, Hikaru's voice has changed to become that dark, dramatic heavy voice type, but not in a
healthy way. If one even watches the early Kalafina music, Hikaru was not always comfortable singing at the top of her range for extended periods of time.
I mean, if Kajiura only wrote "Hikaru no Senritsu"-type music for Hikaru in Kalafina, her voice would have been better preserved as she matured over the years.
For comparison, look at Yuuka Nanri. Hikaru's voice was really only slightly weightier than Yuuka's in my view, and Kajiura had to write mostly to Yuuka's voice type in terms of tempo and melody, while still asking her to sing with more power during their studio sessions! Yuuka was more of a gentle-pop-ballad singer.