Amorphous carbon has emerged as a promising anode material for sodium-ion (SIBs) and potassium-ion batteries (PIBs) due to its high specific capacity, abundant defects, and low production cost. Nevertheless, its practical application remains hindered by suboptimal rate capability and low initial Coulombic efficiency (ICE). Heteroatom doping, particularly with sulfur, has proven to be an effective strategy for addressing these limitations. In this study, sulfur was successfully incorporated in a controlled amount into the amorphous porous carbon framework via a combination of low-temperature carbonization and a facile fumigation process. The introduction of sulfur and the low-temperature treatment synergistically induced a higher density of structural defects, which contributed to significantly enhanced rate performance of the sulfur-carbon (S/C) composite. Furthermore, controlled sulfur doping facilitated the formation of a thinner and more stable solid electrolyte interphase (SEI) during the initial cycling, thereby minimizing irreversible sodium consumption and leading to a substantial improvement in ICE. As an anode for SIBs, the optimized S/C composite delivers a high reversible capacity of 480 mAh g-1 at 0.1 A g-1 and maintains a capacity of 232.6 mAh g-1 even at 5.0 A g-1. Notably, an initial Coulombic efficiency of 83.1% is achieved under a current density of 0.1 A g-1. When applied as the anode for PIBs, the composite exhibits a reversible capacity of 439.2 mAh g-1 and an ICE of 61.2% at the same current density. This work provides a viable and scalable approach to simultaneously enhance the rate capability and ICE of amorphous carbon.