One idea I had recently for a House Rule which would make this a bit more plausible is to give a bonus to lab total / reduction in difficulty for the opening if the traditions are closely related / the existing tradition is a predecessor of the new one.
Right now I haven't got a better idea of how this would work than a ratio of the source of the Breakthrough points, which seems a bit complicated and inconsistent with how other calculations work.
Some sort of approach like this would however be consistent with Birna having difficulty adapting to Hermetic magic - whilst she had contributed, she hadn't contributed that much relative to the large amounts of Mercurian Magic that benefitted most of the other Founders (but she was young and inexperienced enough that it was still possible).
This does leave an outstanding question about Diedne, unless you want to argue that contributing Spontaneous Magic was large enough to give her a significant boost. I don't think we're told how old Diedne was, but I've never got the impression she was particularly junior.
Sub Rosa #16865 AD: Voventes Centennales p.42 has the Minor Hermetic Virtue Cooperative Hermetic Integration for early Bonisagi, replacing their House benefit Puissant Magic Theory.
It derived from talents of Bonisagus, who might have had a stronger version himself.
I did a version of Verditius the founder - reframing him as a faerie-magic practitioner. If I get time I might do the same kind of treatment for Bonisagus.
I don't think he needs, ultimately, a particularly high Magic Theory - the full understanding of its capabilities really come in the generations after him. He needs the ability to initiate others somehow, the ability to teach others despite their pre-existing abilities. So that's where I'd be putting my points/virtues.
You'd also need to be asking the questions around the source of his inspiration. A researcher, fine, but what did he study?
And of course, the question is... What is his Magic Theory? It's not the different types of spell. It's not the ability to bind familiars. It's not the ability to create enchanted devices, or to create longevity potions. It's the Form + Technique understanding.
What was his original tradition? What it is that then lends itself to obtaining that insight?
It may be, actually... Was his invention borne of necessity? Was his control over his own magic so poorly-formed that he had no choice but to develop a new way of looking at it? To compensate?
Interesting problem to crack, old Bonisagus. It may need a couple of new Virtues (and Flaws) to fully model his situation.