Phase-transforming pyroelectric materials have emerged as promising candidates for low-grade thermal energy harvesting. However, whether first-order transformations with large pyroelectric coefficient or second-order transformations with better reversibility are preferable remains unclear. Here we report compositionally tunable phase transformations in Ba1-xSrxTiO3 (x ), revealing progressive weakening of the first-order transformation character with increasing Sr content. We identify a transitional regime between Sr0.15 and Sr0.22 where the transformation becomes more continuous. Within this regime, Sr0.19 achieves optimal lattice compatibility, exhibiting electrical leakage suppressed by over two orders of magnitude while retaining substantial polarization response. Energy conversion demonstrations show the multilayer Sr0.19 device delivers pyroelectric current of ~1.6 uA at 64C with an energy density of 1.6 mJ/cm3 per cycle and 5.5% conversion efficiency. Remarkably, this composition operates stably over 10,000 full energy conversion cycles without external bias field or recharging, demonstrating that transitional regime compositions provide the optimal balance between energy density and operational durability for practical low-grade heat harvesting.