Organic mixed ionic-electronic conductors (OMIECs) are becoming increasingly important for electrochemical applications, particularly bioelectronic devices. Multiple strategies have been investigated to achieve mixed conduction in organic conductors, notably side chain engineering which introduces hydrophilic units onto conjugated backbones. Although this approach has been extensively explored in well-studied conjugated systems, thienoisoindigo (TIG)-based systems remain comparatively underexplored. Here, we focus on TIG as a promising building block and systematically investigate a series of side chain-engineered polymers in aqueous environments. By tuning the ratio of glycolated side chains, we not only establish a structure-property relationship between glycol side chain composition and electrochemical performance, but also demonstrate significantly enhanced ionic conductivity, gravimetric capacitance, and redox activity in resulting devices. Solid state and electrochemical characterizations reveal that these improvements arise from changes in both side-chain-induced morphology and chain reordering. Particularly, the fully glycolated TIG polymer adopts a predominantly edge-on orientation, in contrast to the face-on orientation observed in alkylated analogues, and displays reduced long-range aggregation. These structural features improve ion accessibility and facilitate efficient ion-electron coupling. Device-level characterization using organic electrochemical transistors (OECTs) demonstrates substantial performance gains, with drain current and the figure of merit (μC*) increasing by approximately two orders of magnitude upon intermediate glycolation. However, this enhanced electrochemical performance is accompanied by reduced structural stability, likely due to increased disruption of polymer packing by mobile ions. This study shows that side chain engineering in TIG-based polymers is a promising platform for mixed ionic-electronic conduction, while underscoring the critical balance between performance and stability in OMIEC design, especially in underexplored builiding blocks.