Max supported speed for GL-M2 5G?

I have both the GL-M2 (with RM520GL) and GL-AX3000-5G.

My GL-AX3000-5G usually gets >300 Mbps while GL-M2 tops out at around 2xx Mbps on the same bands/towers. Is there a max supported speed for GL-M2?

I have the GL-M2 connected (via USB3) to GL-AXT1800. Is the USB3 port slowing things down? The USB3 cable is about 1ft or 0.3m.

I’m not sure how helpful this would be but would it be worth tethering the GL-AXT1800 to the GL-AX3000-5G via ethernet and checking the speeds? Not an apples to apples comparison but would be interesting to see if the speeds improve over the M2 as one would expect them to based on what you’ve mentioned so far.