We report a series of novel polymeric mixed ionic-electronic conductors based upon the incorporation of fused thieno[3,2-b]thiophene and bithiophene with isomeric sidechains. The enhanced rigidity in the polymer backbone facilitated the formation of stabilized bipolarons, while the reorganized ethylene glycol side chains not only maintained the polymer's hydrophilicity but also unexpectedly enhanced the crystallinity of the polymer film. This design strategy led to the development of 4gTT-2gT, a high-performance and stable organic mixed ionic-electronic conductor. The polymer exhibited a maximum µC* of 729 F V−1 cm−1 s−1, one of the highest values among low-threshold voltage polythiophene derivatives, while demonstrating excellent operational stability, retaining 99% of its maximum current after 1-h device switching cycles and 94% after 9-h at lower bias. We implemented the material in OECT-based artificial synapses, which maintained functionality under a large temperature range (23–373 K). These combined properties establish 4gTT-2gT as a prime candidate for next-generation mixed conductor devices.