700c (29) is 622mm diameter. 650b is not a halfway house, especially as rotational inertia has a squared relationship with diameter.
Just to point out that inertia will go up in a linear proportion to wheel size as the wheel will be rotating at a lower RPM for a given ground speed.
So rim circumference actually ends up being the key feature (I visualise it as unwrapping the rim and accelerating it along it’s length). Circumference is pi*dia, and a 29″ rim is ~11% larger than 26″, so inertia goes up by ~11%.