In 2023, high-temperature superconductivity with Tc ~ 80 K was reported in the bilayer Ruddlesden-Popper nickelate La3​Ni2​O7​ under ~14 GPa. Two critical issues, however, remained: the nominally "bilayer" phase tends to contain intergrown single-layer and trilayer impurities, and Meissner diamagnetism has rarely been detected, leaving bulk superconductivity unestablished. Partial substitution of La^{3+} by Pr^{3+}/Sm^{3+} has mitigated these problems, yet the superconducting volume fraction of the 80 K phase remains far below unity, necessitating continued material optimization. To accelerate the feedback cycle between synthesis and characterization, we are pursuing high-precision, high-throughput evaluation of the Meissner diamagnetic volume fraction. Using a polycrystalline nano-diamond pressure cell capable of resolving diamagnetic susceptibilities below 1% at pressures exceeding 20 GPa, we present SQUID magnetometry results that quantitatively track the evolution of the superconducting volume fraction across the bilyer nickelate​ series. We also introduce our ongoing development of a diamond NV-center sensing platform for local magnetometry under high pressure, targeting spatially resolved imaging of the inhomogeneous Meissner state.