Have you used the
ChromaRes.png test image to see if you are getting sub-sampled chroma? I assume you are anytime you are not in PC mode. If so I would never go above Jinc for chroma upscaling, and 3-tap spline is probably more appropriate; more is simply wasting power.
I also have noticeably worse banding when sending my TV (LG OLED C7) more than 8 bits in PC mode, have you tried using RGB 4:4:4 8 bit in your AMD settings?