The mesh is a flat polygon,
I am getting the same results when using a completely blank material as well, with just a small roughness set to it:
The rest of the settings you mentioned were already the ones you suggested.
The command line:
r.ssr.experimentaldenoiser 1
r.ssr.quality 4
r.ssr.temporal 1
Did improve the result, but introduced a lot of animated-noise of sorts to the scene