QoS is a huge subject but in relation to the DSL-AX82U and the short version. (don’t hold me to this) …
CAKE QoS: Ensure fairness so one device / data type does not hog all the bandwidth (IP fairness)
Adaptive\Flex QoS: Is rule based which allow your highest priority (most important) applications to continue to function while other don’t when there is not enough bandwidth, lower priority services can suffer.
CAKE QoS is great, as you can set it up and forget it, but if usage exceeds bandwidth allocated, then data streams can suffer.
I use Tradition QoS on my ASUS routers as Gaming PC’s and Consoles are top priority.
FYI: (conversations I have had in the past with ASUS about the DSL-AX82U)
- Traditional QoS on DSL-AX82U use fq_codel
- Traditional QoS disables NAT Acceleration for the DSL-AX82U
- Adaptive QoS does not use fq_codel it uses sfq
- fq_codel generally improve bufferfloat issues compered to sfq
A case of trial and error with QoS, but look at CAKE and Traditional QoS as you first best two options 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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