"Working" QoS (with fc on), and getting those speeds, is noteworthy. The AX86U (not pro) can't do it, and Asus doesn't seem to care.
So, for giggles, I tried enabling Adaptive QoS again, figuring "Why not?"
I've, in the recent past, gone to 500/500 FTTH, but haven't bothered with QoS, due to earlier issues I noted above (and a dwindling use case at 500/500 speeds).
The results on 388.2_2 seem promising, but I'm going to wait a day (and reboot) to confirm more. I also need to confirm that Asus didn't pull a trick and disable things to mask a bug.
From router, before is below:
From an AX wifi client:
BB Pre: https://www.waveform.com/tools/bufferbloat?test-id=67d72e64-dff4-4413-9891-58acbb8448c4
BB Post: https://www.waveform.com/tools/bufferbloat?test-id=fd2c7508-0d74-4280-9236-d9d01cb6a600