A model is constructed to predict the emission originating from axion-to-photon conversion in the strongly magnetized ultrarelativistic plasma of neutron stars. The acceleration and multiplicity of the charges are observed to shift the axion-induced spectral feature with respect to previous expectations. The frequency range of interest widens accordingly, and heavier dark matter axions may resonate in magnetospheric splits giving rise to detectable radio signals that could extend into the millimeter band. Ultimately, this work follows an affirmative answer to the question of whether neutron stars can give rise to any detectable high-frequency spectral feature that would allow us to probe axion dark matter of masses up to about a millielectronvolt. SGR 1745–2900 emerges as a particularly promising astrophysical laboratory for probing high-frequency axion dark matter.