What structural feature characterizes the smallest red dwarfs, enabling almost total hydrogen consumption?

They are fully convective

The internal structure of the smallest red dwarfs differs significantly from that of larger stars like the Sun. In a fully convective star, the material within its interior is constantly churning and mixing, moving energy via convection currents from the core outward. This continuous mixing prevents the buildup of inert helium, which is the byproduct of hydrogen fusion, in the central region. Because the helium ash is transported outward and fresh hydrogen is brought into the fusion zone, these stars can utilize nearly 100% of their initial hydrogen fuel supply over their immense lifespans, leading to lifespans up to 10 trillion years.