For optimizing steady-state performance in organic electrochemical transistors (OECTs), both molecular design and structural alignment approaches must work in tandem to minimize energetic and microstructural disorders in polymeric mixed ionic-electronic conductor films. Herein, a series of poly(diketopyrrolopyrrole)s bearing various lengths of aliphatic-glycol hybrid side chains (PDPP-mEG; m = 2-5) is developed to achieve high-performance p-type OECTs. PDPP-4EG polymer with the optimized length of side chains exhibits excellent crystallinity owing to enhanced lamellar and backbone interactions. Furthermore, the improved structural ordering in PDPP-4EG films significantly decreases trap state density and energetic disorder. Consequently, PDPP-4EG-based OECT devices produce a mobility-volumetric capacitance product ([µC*]) of 702 F V^-1 cm^-1 s^-1 and a hole mobility of 6.49 ± 0.60 cm² V^-1 s^-1 . Finally, for achieving the optimal structural ordering along the OECT channel direction, a floating film transfer method is employed to reinforce the unidirectional orientation of polymer chains, leading to a substantially increased figure-of-merit [µC*] to over 800 F V^-1 cm^-1 s^-1 . The research demonstrates the importance of side chain engineering of polymeric mixed ionic-electronic conductors in conjunction with their anisotropic microstructural optimization to maximize OECT characteristics.