Hematite (α-Fe₂O₃) is a prototypical antiferromagnetic platform for high-speed spintronics and magnonics, yet its finite-momentum magnon dynamics and spin-reorientation mechanism remain incompletely understood. Here we use low-wavenumber magneto-Raman spectroscopy to resolve both k=0 and finite-k sub-terahertz magnons in α-Fe₂O₃ across the Morin transition. We find that the limiting group velocity v₀ remains nearly field independent, whereas the finite-k group velocity v_g is strongly modified near the Morin and spin-flop transitions. By combining parallel- and transverse-field measurements with spin-wave modeling, we further extract the temperature-dependent Dzyaloshinskii–Moriya field H_D and uniaxial anisotropy fields H_K1 and H_K2. We find that H_D is weakly temperature dependent at approximately 2.0–2.3 T, while H_K1 decreases rapidly and H_K2 changes only weakly. This contrast drives the sign reversal of the effective anisotropy field and quantitatively accounts for the Morin transition. Our results establish low-wavenumber Raman spectroscopy as a quantitative probe of finite-momentum antiferromagnetic magnons and provide key material parameters for hematite-based magnonics.