Accurate real-time monitoring of early seismic deformation using GNSS has long been limited by the coupling of multiple geodetic parameters, producing spurious co-seismic signals during the initial phase of earthquakes. Here, we present a full decoupling framework driven by models that separates satellite ambiguities, ionospheric and tropospheric delays, clock offsets, and position estimates. Application to the early phase of seismic events shows that fluctuation errors at the centimeter level in co-seismic deformation are reduced to the millimeter scale, with accuracy approaching that of seismic accelerometer measurements. Even after regional augmentation and ambiguity resolution have been applied, we still found a false co-seismic waveform with an amplitude of up to 3 cm during the early phase of the earthquake. This results from noticeable parameter coupling, with MAD values of correlation coefficients exceeding 0.26 between ionospheric delays and both the vertical position and receiver clock offset parameters. By fully decoupling these parameters, the decoupling framework effectively corrects the deformation artefacts introduced by ionospheric errors, thereby enhancing detection of seismic and ionospheric processes during the main shock.