My only objections to choosing Bjornaer first for a math-heavy mystery cult are a) they are already in a mystery cult, and some members of the House may have an issue with joining more than one^ and b) they spend a lot of time in a form that cannot do math. A vast majority of noble beasts lack the finger-equivalents and manual dexterity to scratch out complex calculations in the dirt.
It has nothing at all about the cliche about uneducated and grubby Bjornaer wearing badly tanned furs, mostly speaking at tribunal in grunts. It had entirely to do with cult exclusivity and not having any damn hands. Nerd out, journey to Islamic lands to find the best armillary spheres possibles, wear immaculate clothing and speak with a noble accent. It's your character, enjoy fantastic stories, have fun. But max out your Finesse if you want to use unseen arm and a handy stick to calculate horoscopes.
If you really want to create a new virtue for your new cult, invent one that allows the Bjornaer to transform into their heartbeasts with their carried possessions, like almost every muto effect. The sign up line would go around the corner. Also: the Irish will be puzzled as hell if you call a different group "milesians". Language is funny like that.
^ The Huntress cult is mostly comprised of a small minority descending from a faction of original Merinita, who joined the Bjornaer when Quendalon changed the focus of the House, and their pagan worship of Diana will not go over well if discovered. Since they are a sub-set of an already small group--and the entirety of House Bjornaer has less than 100 members in the world--the Huntress cult will barely have enough members for a decent D&D game.