The management and valorization of sewage sludge remain important environmental challenges, particularly in arid regions with low soil organic matter. This study addressed this issue by evaluating selected thermotolerant microorganisms during 90-day sewage sludge composting under the conditions of the Mangystau Region, Kazakhstan. Throughout the experiment, composting dynamics, compost maturity, nutrient transformation, and plant responses were assessed. The inoculated treatments showed more pronounced compost maturation than the uninoculated control. The combined Bacillus–actinomycete treatment was associated with more pronounced stabilization of organic matter and the most favorable plant response, with humus content reaching 20.0% by day 90. Compost maturation was also accompanied by reduced phytotoxicity and improved plant development, particularly in the inoculated treatments. Overall, the findings demonstrate the potential of selected thermotolerant Bacillus strains and actinomycetes for sewage sludge bioconversion and stabilization, supporting further evaluation of the resulting material as a potential organic soil amendment for use under arid conditions.