The theoretical maximum depends on the client device (handheld/battery-powered vs. AC wall-powered computer on 'best performance settings'). It also depends on the distance tested. What is used for the testing, and is still only the connection speed, not the actual throughput possible to be achieved.
Disabling WiFi 6/AX mode and using 40MHz wide channels is also lowering the throughput. As (possibly) is leaving the Control Channel to Auto.
So, what are you using to test with? What is the indicated connection rate? How are you testing? What kind of environment are you in (with regard to WiFi)?
That's the most you will get because you have disabled ax mode and set the bandwidth to 20MHz. To get 1148Mbps you need to use ax mode, have a bandwidth of 40MHz and use an ax client with four antennas.
That depends entirely on your local situation. Nobody have can tell you which is "better", you'll have to test it for yourself. That said, I would start with 20/40 and see how that goes.
In reality with AX mode disabled on 2.4GHz - 72Mbps link speed (~50Mbps throughput) to 1-stream N client and 144Mbps link speed (~90Mbps throughput) to 2-stream N client @20MHz wide channel. 20/40 setting almost guaranteed fails back to 20MHz over time, unless the house is in the middle of a corn field. Forced 40MHz wide channel is not recommended - it takes almost the entire 2.4GHz spectrum and often in (normal) interference conditions works worse than 20MHz wide channel.
