I managed to puzzle a bit within the RetroArch menu (hotkey+north within the game), inside the core options, and restarting ES after each change. Might polish a bit more.
It came down to assuring the 1x resolution and some extra texture options for preference. It doesn't beat Mupen64plus graphics yet, but it is damn close right now.
I could also push the 4:3 top and bottom scaling within the glideN64 core options, no more black borders.
I am 95% satisfied! 🥂🙏🏻🎮