Transition-metal (TM) substitutional defects in cubic silicon carbide (3C-SiC), which host spin-1/2 ground states with $d$-orbital character under ${\mathrm{T}}_{\mathrm{d}}$ symmetry, are emerging candidates for quantum applications. Their pseudospin parameters, including $g$ factor and effective spin-orbit splitting, are central to electron-spin resonance and quantum control. These parameters arise from the competition between spin-orbit coupling and dynamic Jahn-Teller (DJT)-mediated electron-phonon coupling. However, the microscopic mechanism governing this competition and its evolution across different TM defects remain poorly understood, severely limiting the rational engineering of these defects. Using the hybrid functional with the ${\mathrm{V}}_{\mathrm{w}}$ correction scheme (HSE + ${\mathrm{V}}_{\mathrm{w}}$), we present a systematic first-principles study of spin-1/2 $\mathrm{T}{\mathrm{M}}_{\mathrm{Si}}\left(\mathrm{TM}=\mathrm{V},\mathrm{C}{\mathrm{r}}^{+},\mathrm{M}{\mathrm{o}}^{+},{\mathrm{W}}^{+}\right)$ defects in 3C-SiC. The experimental $g_{\parallel }$ of ${\mathrm{V}}_{\mathrm{Si}}$ is reasonably captured and its deviation from the free-electron value can be attributed to covalency and DJT-mediated Ham reduction. The pseudospin parameters of group-VIB $\mathrm{T}{\mathrm{M}}_{\mathrm{Si}}$ defects are further predicted and we find that DJT-mediated electron-phonon coupling anomalously strengthens with increasing atomic number—opposite to the trend in group-IV vacancy color centers in diamond—causing the Ham reduction factor to decrease from Cr to Mo and W. This anomalous enhancement competes with the expected growth of spin-orbit coupling, substantially suppressing the growth of effective spin-orbit splitting and the $g$ factor deviation across the series. Our results establish DJT-mediated electron-phonon coupling as a key modulator of spin-orbit-driven pseudospin properties in TM defects of diamondlike materials.

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• Defects
• Jahn-Teller effect
• Spin-orbit coupling
• Zeeman effect
• Spin
• Density functional calculations