Sr₂TiSi₂O₈ (STS) polar glass-ceramics are promising piezoelectric materials for high-temperature surface acoustic wave (SAW) devices, provided that the orientation of the STS polar axis and the thermomechanical stability of the substrate can be controlled. In this work, the effect of the residual glass fraction on the crystallization behavior, microstructure, and thermomechanical properties of glass-ceramics synthesized from parent glasses belonging to the Sr–Ti–Si–K–Al–O system was investigated. Dilatometry, differential scanning calorimetry, and controlled heat treatments performed on the parent glasses showed that increasing the residual-glass-forming fraction slows the crystallization kinetics of STS. All investigated compositions exhibited a surface-crystallization mechanism, and the growth rate of the crystallized layer at 900 °C decreased with increasing residual glass content, thereby affecting the preferential orientation of STS crystals. The thermomechanical behavior of the resulting glass-ceramics was investigated by dilatometry and by measuring the temperature dependence of Young’s modulus using the impulse excitation technique (IET). The results were analyzed with reference to the behavior of both the residual glass and pure STS. Increasing the residual glass fraction only slightly affected the coefficient of thermal expansion (CTE) below the glass transition temperature (Tg) of the residual glass but led to a significant increase in CTE at higher temperatures. Nonlinear thermal-expansion behavior, attributed to the commensurate–incommensurate (C–I) phase transition of STS, was observed in the temperature range 100–200 °C. This phase transition also affected the evolution of Young’s modulus with temperature. Furthermore, the combined dilatometry and IET results suggest that residual stresses generated during crystallization contribute to a downward shift in the apparent commensurate–incommensurate (C–I) transition temperature of STS.