I fixed it!
https://www.youtube.com/watch?v=pk7hONu0b1c
This video explained it all. The other documentation was either unclear or the process was not intuitive.
For example I needed to make the firewall rule on the LAN and use the source IP. I assumed the WAN is needed to be limited to a destination address - and that didn't work. I also needed traffic limiter entries for In and Out.
It worked for the videos. 10Mbps still invoked HDR for Amazon but 6Mbps brightened the picture considerably. I will experiment with other speeds a bit later.