Using a combined approach of first-principles and Boltzmann transport theory, we conducted a systematic investigation of the thermal and electrical transport properties of the unexplored ternary quasi-two-dimensional KMgSb system of the KMgX (X=P,As,Sb, and Bi) family. In this paper, we present the transport properties of KMgSb under the influence of hydrostatic pressure and alloy engineering. At a carrier concentration of ∼8×1019cm-3, we observed a close match in the figure of merit (zT; ∼0.75, at 900 K) for both n-type and p-type KMgSb, making it an attractive choice for engineering thermoelectric devices with uniform materials in both legs. This characteristic is particularly advantageous for high-performance thermoelectric applications. Additionally, as pressure decreases, the zT value exhibits an increasing trend, further enhancing its potential for use in thermoelectric devices. Substitutional doping (replacing 50% of Sb atoms with Bi atoms) resulted in a significant ∼49% (in-plane) increase in the peak thermoelectric figure of merit. Notably, after alloy engineering, the maximum figure-of-merit value obtained reached ∼1.45 at 900 K temperature. Hydrostatic pressure emerges as an effective tool for tuning the lattice thermal conductivity κL. Our observations indicate that negative-pressure-like effects can be achieved through the chemical doping of larger atoms, especially when investigating κL properties. Through our computational investigation, we elucidate that hydrostatic pressure and alloy engineering hold the potential to dramatically enhance thermoelectric performance in this compound.