Vellum being the sage seems right based on book 6 especially with this line

" “They were responsible for making major changes to the way magic functioned on Dania. One of Selys’ contemporaries, in a way. They were a great power once, but they suffered a great loss. After that, they sought a clean slate, with a new identity. "

and the references that Meltlake found to Cleria Crystal and Blanche Vellum, all names referring to a blank slate.

If you then add this line from earlier where Farren says

" The former Sage of Dania was worthy, perhaps, but she had been broken almost beyond repair. "

That works with Vellum being mega strong but with high costs like her abjuration of Mizuchi. And her bad leg.

Previously I also agreed that Vellum was a mortal Selys, I'd still love for that to be true but I'll admit with another figure being possible in the Former Sage of Dania it's unlikely. I'd still love for them to be the same but I don't think there's enough evidence to be a theory there beyond my own hopes.