I'm running 18.06.5 on an R7800 and followed the instructions at this wiki page to improve bufferbloat. Interestingly, using 95% or 90% of my max upload/downstream really limits the downstream bandwdith.
For example, without the QoS at all, I get an ave down of 210,000 kbit/s and an ave up of 12,720 kbit/s.
When I use values of 95% or 90% of these max values, the best downstream I can achieve is 160,000 kbit/s ... that is using values of 196,935 or 186,570 for the download. Is that normal? I also tried using a 0 for the download and am finding the same wall around 160,000 kbit/s.
Other settings are as explained on the wiki page, but in summary:
queue discipline = cake
setup scripts = piece_of_cake.qos
per packet overhead = 22 (but also tried 44 with no change)