The present research has made an effort to support the subgrade strength performance of polyvinyl alcohol fiber-reinforced (PAF) soft soil stabilized with an alkali-activated stabilizer (AAS). The research compares the subgrade stiffness parameters (resilient modulus, penetration, and flexural resistance) between hydrated lime and alkaline stabilizer mixed PAF-soil at various slag/bagasse ash (BA) proportions. The influences of slag, BA, and PAF changing percentages in the AAS-soil matrix on the California Bearing ratio (CBR) and soil-beam flexural strength (Sf) are evaluated. The study also proposes an optimum dosage of slag-BA-PAF in alkaline stabilized soft subgrade soil. It is observed that PAF-AAS soil exhibits higher interfacial interactions and inter-bonding density than lime-PAF-soil under low tensile cracking. The combined PAF (0.6%) with 20% BA replacement with slag in the alkaline stabilizer strengthens the CBR and flexural strength by around 45% over lime soil-fiber mixtures. Moreover, a detailed reliability index (β) was also conducted to evaluate further the impact of uncertainties in the PAF-AAS-soil mixture on subgrade performance. The research study shows that the higher percentage of PAF-slag in a BA-based AAS mixture can substantially improve the stiffness and durability of soft subgrade soil.