Found this run-once app Hide Navigation Bar by Manuel Wrage that hides the navigation bar in the S2 Oreo 8.0 or Treble 4.1. Once ran, can delete it and the navigation bar removal persists.
After running, it allows exclusive use of the fingerprint sensor (FP) for navigation (tap=back, long-tap=home, swipe=apps). Treble 4.1 with an additional double-tap=lock phone.
In Treble 4.1, it removes the bottom corner rounding and just leaves the top rounded corners. Nice. Betters setting qemu.hw.mainkeys=1 in build.prop which also hides the navigation bar on Treble 4.1 but maintains bottom corner rounding.
If could get landscape mode with Chrome showing info around the notch, would use Treble 4.1. It's impressively good. Great work @heineken78