It's really sad to say but unfortunately, Twitch is turned to nVidia NVENC encoder and doesn't give a f* to improve the streaming quality for AMD cards. Basically, if you plan to stream only on Twitch, you're fated to have bad image quality when gaming or streaming any fast movement, it is still good for static images but we know it's usually not the plan... Another option is using a streaming PC with NVENC. 6000 series doesn't have AV1 so we're in the dark.
Edit: another point to consider, your upload rate is quite low for streaming, maybe you're getting very bad quality to stream under 10Mbps than the issue with the codec itself.
