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We present a systematic, multiparameter study of Rb/(129)Xe spin-exchange optical pumping (SEOP) in the regimes of high xenon pressure and photon flux using a 3D-printed, clinical-scale stopped-flow hyperpolarizer. In situ NMR detection was used to study the dynamics of (129)Xe polarization as a function of SEOP-cell operating temperature, photon flux, and xenon partial pressure to maximize (129)Xe polarization (PXe). PXe values of 95 ± 9%, 73 ± 4%, 60 ± 2%, 41 ± 1%, and 31 ± 1% at 275, 515, 1000, 1500, and 2000 Torr Xe partial pressure were achieved. These PXe polarization values were separately validated by ejecting the hyperpolarized (129)Xe gas and performing low-field MRI at 47.5 mT. It is shown that PXe in this high-pressure regime can be increased beyond already record levels with higher photon flux and better SEOP thermal management, as well as optimization of the polarization dynamics, pointing the way to further improvements in hyperpolarized (129)Xe production efficiency.