Nitrogen-vacancy centers in diamond allow coherent spin state manipulation and optical readout at room temperature, which has powerful applications in nanoscale sensing. Nanophotonic structures such as plasmonic waveguides, nanoantennae, metamaterials, and metasurfaces can enhance the detected fluorescence rate from such broadband emitters. The fluorescence of the coupled emitter is directed into confined plasmonic modes with high photonic density of states. However, an accurate spin readout requires both high photon counts and a strong contrast between the spin states, both of which can be influenced by the Purcell effect. We introduce a novel method for measuring the spin contrast in large nitrogen-vacancy ensembles. We use this method to study how the photonic density of states must be engineered in order to minimize the uncertainty of spin readout in dense NV ensembles. We describe these results using a kinetic model of the nitrogen-vacancy's internal dynamics.