We propose a high-precision x-ray spectroscopy experiment of antiprotonic atoms to advance the understanding of low-energy antinucleon–nucleus interactions. The current leading model of antiproton–nucleus interactions is based on an optical potential with parameters derived from a global fit to antiprotonic atom x-ray data across the periodic table. However, the isovector parameter of this potential remains poorly constrained due to uncertainties in nucleon distributions of the nuclei. To address this, we propose to use calcium isotopes with well-studied nucleon distributions to minimize these uncertainties. A superconducting microcalorimeter detector will provide a resolution of 50–70 eV in the energy range of interest, allowing high-precision determination of the isotope-dependent strong-interaction shifts and widths. The outcomes of the proposed experiment can be used to refine the model of antinucleon-nucleus interactions and provide critical data for future experiments searching for neutron-antineutron oscillations.