If anyone remembers Jedi Boadicea's "Frozen Sky", I'd like to point to it as a previous example of a very good Hitsugaya Toushiro backstory fanfiction.
"Lord of Winter" is faintly reminiscent of "Frozen Sky" in style. Cymru's prose is poetic and fluid, full of vivid imagery and beautiful descriptions that read almost like a stream of consciousness at times. There's a great deal of character introspection, and all of it is well handled and delivered in a believable way.
What really makes "Lord of Winter" stand out, however, is the painstaking attention to detail. Kubo Tite has a bad habit in canon of introducing new elements without ever fully explaining them. "Lord of Winter" goes through the brief mentions of things like the Shinigami Academy and fleshes them out, giving them depth and making them so believable they could be real places. Even combat concepts like zanjutsu, hoho, hakuda, and kido are explored and explained in a way that canon never does.
Personally, I found the pacing perfectly acceptable for this style of storytelling, but Your Mileage May Vary. Cyrmu spends a lot of time explaining details and exploring the nuances of her characters' states of mind, which might make the story seem slow to more action-hungry readers. However, for readers that like seeing characters and worlds fully fleshed out and explored, this story is a rare treat.
All in all, I highly recommend giving "Lord of Winter" a try, even if you're not a Hitsugaya fan. It certainly helps you see Bleach-verse in an entirely new light.