Addressing the demands of global waste heat recovery and low-power spin-based information processing, this paper designs and investigates two types of molecular-scale nanodevices composed of the two-dimensional ferromagnetic material Fe₃GeTe₂ and the organic molecule C₈-BTBT with different interfacial coupling modes, based on first-principles density functional theory (DFT) and the non-equilibrium Green's function (NEGF) method, aiming to reveal the influence mechanism of interfacial coupling strength on the spin thermoelectric transport properties of the devices. The results show that the interfacial coupling strength directly affects the interfacial charge transfer and spin transport behavior: the charge transfer amount is 0.766 e^- in the strong coupling mode, while it decreases to 0.256 e^- in the weak coupling mode. Under the strong coupling mode, the device exhibits high electrical conductance and thermal conductance, whereas under the weak coupling condition, both charge transport capability and thermal conductance are significantly reduced. Due to the trade-off between electrical conductance and thermal conductance in thermoelectric performance, the thermal conductance is markedly suppressed under weak coupling, leading to an increase in the spin figure of merit (ZT_sp) from 0.71 to 3.45. Notably, the study also reveals that interlayer coupling can modulate the distribution of spin charge density, resulting in a significant polarity reversal of the spin current in the molecular device under different coupling modes. In addition, the thermally induced negative differential resistance effect is observed. This study provides a theoretical basis for the development of high-efficiency and tunable spin thermoelectric prototype devices through interfacial coupling engineering.