I’ve never been able to sync faster than 150 Mbps with a cheap USB WiFi dongle (I have half a dozen, each with different chipsets used for various geeky projects).
Even if the wireless speed is good enough, there can be other bottlenecks because of network interface, USB interface limits and all sorts.
The reason for the Luci issues may be that your router is using a proprietary driver for the WiFi which does not make these information available to luci. You can try pure open source version of router for different WiFi features with open source drivers at the cost of some performance.
You are being a little stubborn on this issue (not to be rude) - you hsould really ask yourself if this is worth it.
FWIW I’m pretty sure that I connected with 300 Mbps sync with my laptop to the mt300 v2. I don’t have it anymore to test and modern firmwares block 40mhz when there are neighboring wifis (as it should).
At this stage it’s probably not worth the hassle - with those 4 WiFi adapters and the mango, you could have just bought a 5ghz enabled Slate or Creta.