I don't have an Mi 11 ultra nor the Ax88U.
What I have is an Mi 11 and an AX92U also an AVM 6660 Cable, on both set to Wi-fi 6 I get an dl speed of over 128 MB/s.
Not yet but once I'm home I can.. I already see some problems in yours. Enable 160 MHz to start with. Also since you set 5 GHz only for the SSID 20/.. MHz setting doesn't make much sense.
This is maybe not a "personal" problem. If we use the same chipset and config on the Asus routers, and the Ultra model can't get the speed over AX, but the normal 11 Pro does, we have not a personal problem here.
I could fix the Problem. I had to disable "802.11ax/ac Beamforming" and "Universal Beamforming". Now i can reach max. speed of my home internet 500/50.