Adorama has the highly-rated Tokina Opera EF 16-28mm F/2.8 FX for $549 (Reg $699). This is an EF lens that will obviously work with your EF to RF adapter for use on your EOS R camera.
Zoom Focal Length: 16-28mm Lens
Maximum Aperture: f/2.8
Image Stabilization Type: No
Lens Series: Tokina FX Series, Tokina opera Series
Special Features: Aspherical
Tokina Opera EF 16-28mm F/2.8 FX $549 (Reg $699)