We present a comprehensive first-principles investigation of the electronic structure and excited-state carrier dynamics in the carbide antiperovskites Ca6CSe4 and Sr6CSe4. Using many-body perturbation theory (G0W0/BSE), we show that both materials are direct band gap semiconductors with quasiparticle gaps of 1.66 eV (Ca) and 1.22 eV (Sr), lying in the visible-near-infrared range, and exhibit moderate excitonic binding energies. Ab initio nonadiabatic molecular dynamics simulations at 300 K reveal distinct relaxation mechanisms governed by the interplay of band gap, nonadiabatic (NA) couplings, and electronic decoherence. In Ca6CSe4, stronger lattice fluctuations induce 38% larger band gap variations and 28% faster decoherence, which, together with approximately 53% weaker NA couplings, suppress nonradiative recombination and yield lifetimes nearly 18 times longer (40.3 ns) than in Sr6CSe4 (2.2 ns). Hot-carrier cooling in both systems occurs on picosecond timescales (1-9 ps) with a pronounced slow down near the band egdes. Overall, our results demonstrate that A-site cation substitution provides an effective route to control carrier lifetimes and relaxation pathways in antiperovskites, offering microscopic insight into lattice-driven carrier dynamics and guiding their experimental realization and optimization.