To see what your wifi is capable of, do not run iperf3 on the router but run it on a local wired lan client.
Running iperf3 on the router really bogs the router down.
I am far from a wifi expert and testing wireless throughput is difficult, what kind of client how many streams, bandwidth, modulation, polluted spectrum, distance etc. etc.
Usually I see testing done with 4 parallel streams -P4
But a good place to start is looking at the connection of your client in Status > Overview
make sure your client is active otherwise it will fall back.
Attached picture of my Samsung S20 connected to my router, 5 GHz AX, 80 MHz clean spectrum close to router.
The PHY rate is indeed 1200 Mb/s which is the max for AX routers (Dynalink DL-WRX36) with these settings (for AC it is 866 Mb/s).
Actual iperf throughput is of course way less usually about 2/3 of the PHY rate.
In my case I measured about 700 Mb/s with iperf3 -P4
