Sidr (Ziziphus spina-christi L.) is an underutilized fruit native to arid and semi-arid regions that possesses considerable nutritional and phytochemical value. However, its potential for controlled lactic fermentation and development into value-added fermented products has received limited scientific attention. This study investigated the effects of five brine formulations on the controlled fermentation of Sidr fruit pickles and monitored changes in physicochemical properties, bioactive compounds, microbial dynamics, texture, color, and sensory attributes during 90 days of storage at ambient temperature. The treatments consisted of 10% NaCl (control), NaCl supplemented with sodium sorbate, NaCl with sucrose and vinegar, NaCl with sucrose and Lactobacillus plantarum starter culture, and NaCl with sucrose, vinegar, and garlic. Brine formulation significantly influenced fermentation kinetics, microbial succession, and product quality throughout storage. The inoculated treatment containing L. plantarum exhibited the most rapid acidification, reaching a pH of 4.02 and titratable acidity of 0.24%, while maintaining the highest lactic acid bacteria population (>9 log CFU g−1) and enhanced microbiological stability. This treatment also showed superior retention of ascorbic acid, total phenolic compounds, antioxidant activity, and texture compared with the non-inoculated treatments. Pearson correlation analysis and principal component analysis (PCA) further demonstrated strong associations between starter-culture fermentation, bioactive compound preservation, and overall product quality. Sensory evaluation indicated that all treatments remained acceptable throughout storage; however, the inoculated samples consistently received the highest scores for taste, texture, and overall acceptability. Overall, the results indicate that controlled lactic fermentation using L. plantarum represents an effective approach for enhancing the quality, stability, and bioactive retention of fermented Sidr fruit products, supporting the valorization of this underexploited fruit resource for sustainable food applications.