I take the point in that Brian going “I’m Susan now” is insufficient in and of itself to classify as TG for practical purposes
What you’re describing would be sufficient for Brian legally to become a woman. Brian wouldn’t need to change her name or her appearance or alter her body in any way. As a self-identifying woman, biologically-male Brian would be free to access women’s refuges, changing rooms, bathrooms, hospital wards, rape crisis centres, healthcare provision (eg cervical screening) and to participate in sport as a woman. If Brian commits a crime statistically she’ll do so as a woman (even if the crime is rape) and serve her sentence in a women’s prison.
What could possibly go wrong?