Stress accumulation on the plate interface of subduction zones is a key parameter that controls the location, timing and rupture characteristics of earthquakes. The diversity of slip processes occurring in the megathrust indicates that stress is highly variable in space and time. Based on GPS and InSAR data, we study in depth the evolution of the interplate slip-rate along the Oaxaca subduction zone, Mexico, from December 2016 through August 2020, with particular emphasis on the pre-seismic, coseismic and post-seismic phases associated with the June 23, 2020 Mw 7.4 Huatulco earthquake to understand how different slip processes contribute to the stress accumulation in the region. Unlike two time-invariant interplate coupling models previously proposed for the region, our results show that continuous changes in both the stress-releasing aseismic slip and the coupling produced a high stress concentration (i.e. Coulomb Failure Stress (CFS) of 700  100 kPa) over the main asperity of the Huatulco earthquake and a stress shadow zone in the adjacent updip region (i.e. shallower than 17 km depth with CFS around -500 kPa). These findings may explain both the downdip rupture propagation (between 17 and 30 km depth) and its impediment to shallower, tsunamigenic interface regions, respectively. Interplate coupling time variations in the 2020 Huatulco and the nearby 1978 (Mw 7.8) Puerto Escondido rupture zones clearly correlate with the occurrence of the last three Slow Slip Events (SSEs) in Oaxaca far downdip of both zones, suggesting that SSEs are systematically accompanied by interplate coupling counterparts in the seismogenic zone that in turn have their own potentially-seismogenic stress and frictional implications. In the same period, the interface region of the 1978 event experienced a remarkably high CFS built-up of 1,000-1,700 kPa, half imparted by the co-seismic and early post-seismic slip of the neighboring Huatulco rupture, indicating large earthquake potential near Puerto Escondido. Continuous monitoring of the interplate slip-rate thus provides a better estimation of the stress accumulation in the seismogenic regions than those given by time-invariant coupling models and improves our understanding of the megathrust mechanics where future earthquakes are likely to occur.