Context. Ultra-hot Jupiters showcase one-of-a-kind extreme atmospheric conditions, including the dissociation of molecules into atomic species, ionisation, and significant day-to-night temperature contrasts. The proximity to their host stars exposes ultra-hot Jupiters to intense stellar irradiation, enabling high temperatures that drive noteworthy contributions to the overall opacity by hydride ions (H‑), potentially obscuring features of metals in the near-infrared transmission spectrum. Aims. This work aims to detect atomic, ionic, and molecular species in the atmosphere of WASP-189b (H, He, Fe, Ti, V, Mn, Na, Mg, Ca, Cr, Ni, Y, Ba, Sc, Fe+, Ti+, TiO, H2O, CO, and OH). A focus is placed on (i) understanding the role of H‑ as a source of absorption continuum opacity, and (ii) retrieving the relative hydride-to-Fe abundance using combined optical and near-infrared data. Methods. We present two transits of WASP-189b gathered simultaneously in the optical with HARPS and in the near-infrared with NIRPS, supported by photometric light curves from EulerCam and ExTrA. Transmission spectra were analysed via cross-correlation to detect a planet's absorption features and to increase the signal-to-noise ratio of potential detections. Additionally, atmospheric retrievals quantified relative abundances by fitting the overall metallicity, and abundance proxies for TiO, H‑, and e‑. Results. Only atomic iron is detected in HARPS data (S/N~5.5). However, no Fe is detected at near-infrared wavelengths, likely due to the H‑ continuum dampening. Atmospheric retrievals on HARPS only and HARPS+NIRPS combined suggest that the hydride-to-Fe ratio exceeds equilibrium model predictions by ~0.5 dex, hinting at a strong ionisation rate for hydrogen atoms. Including NIRPS data helps to constrain the H‑ abundance, as well as set an upper limit on the free electron density, which is unconstrained from the HARPS-only retrieval. These results emphasise the impact of H‑ as a non-negligible source of continuum absorption opacity impeding the detection of planetary absorption features in the near-infrared transmission spectrum of WASP-189b.