  We investigate the spin-coherence decay of NV$^-$-spins interacting with the
strongly-coupled bath of nitrogen defects in diamond layers. For thin diamond
layers, we demonstrate that the spin-coherence times exceed those of bulk
diamond, thus allowing to surpass the limit imposed by high defect
concentrations in bulk. We show that the stretched-exponential parameter for
the short-time spin-coherence decay is governed by the hyperfine interaction in
the bath, thereby constraining random-noise models. We introduce a novel method
based on the cluster-correlation expansion applied to strongly-interacting bath
partitions. Our results facilitate material development for quantum-technology
devices.
