Drying municipal sewage sludge represents a major energy bottleneck in biomass valorization, and conventional thermal methods often strip away its valuable nutrients. This research explores microwave-assisted drying as a strategy to enhance energy efficiency while preserving the carbon-to-nitrogen-to-phosphorus (C:N:P) stoichiometry critical for agricultural applications. Experimental trials evaluated sludge cakes across a broad spectrum of microwave powers (160–800 W) and various geometric configurations. The results revealed a consistent three-stage drying process, characterized by a distinct kinetic shift once moisture dropped below a 50% threshold. Below this point, internal steam generation created a strong pressure gradient—often referred to as the “beggar's chicken effect”—that drove thicker samples to dry more rapidly than thinner ones. A power setting of 480 W paired with a 2.5 cm cake diameter emerged as the optimal configuration, achieving a true thermal efficiency of 49% and a specific energy consumption of just 1.92 MJ/kg. This performance outpaced conventional hot-air and infrared drying methods, achieving a 34% lower specific energy consumption than hot-air drying. Crucially, the rapid, volumetric heating provided by microwaves mitigated the nitrogen loss typically triggered by prolonged thermal exposure. Consequently, the microwave-dried sludge maintained a C:N ratio of 11, closely mirroring the raw feedstock and ensuring high agronomic value. Ultimately, these findings provide a practical, energy-efficient pathway for transforming biosolids into premium bio-fertilizers and bioenergy feedstocks within circular economy models.