Realizing well-defined organic mixed ionic-electronic conductors (OMIECs) with more delocalized electronic structures remains challenging in intelligent and wearable bioelectronics. Here, we present a metal-free, step-economical π-extension strategy to construct biocompatible, monodisperse, fused all-acceptor oligomeric OMIECs (ONRs) for intelligent ion sensing. This approach established a clear relationship between molecular structure, microstructure, and device function. As one of the longest reported monodisperse OMIEC (45.7 Å), 4NR delivers vertical organic electrochemical transistors (vOECTs) with high transconductance (7.42 mS), ultrafast switching (90% retention), arising from its optimized microstructural balance that promotes ion accessibility while preserving long-range electronic connectivity. By leveraging voltage-dependent, reconfigurable n-type and anti-ambipolar behaviors, a waveform-based, non-invasive intelligent sweat-ion sensing platform integrated with machine-learning classification is established, enabling accurate discrimination of ionic concentration levels across physiologically relevant ranges, further highlighting the promise of precisely π-extended OMIECs for intelligent wearable bioelectronic sensing.