Sorry for the late reply.
My router is the Linksys WRT1900ACSv2
I have been testing with the Android Ookla Speedtest and Magic iPerf apps on a Samsung S8. I first disabled SQM for my initial tests and did half a dozen tests on the same server to ensure I was reaching maximum speeds that are just over 300Mbps on 5GHz.
SQM disabled
Ookla Speedtest = 300Mbps
iperf3 = 442Mbps
Minimum idle = 63%
Average idle = 70%
I then checked the Enable this SQM instance checkbox on the sqm-qos page and selected pppoe-wan (wan,wan-6) from the drop-down menu, clicked the save & apply button and retested on the same server. The download speeds now capped out at 170Mbps.
pppoe-wan (wan,wan_6)
Ookla Speedtest = 170Mbps
iperf3 = 434Mbps
Miniumum idle = 71%
Average idle = 75%
To ensure the test server wasn't being slow and giving me false readings, I immediately went back to the sqm-qos page, unchecked the checkbox, clicked the save & apply button and retested. The speeds went straight back up to 300Mbps.
SQM disabled
Ookla Speedtest = 303Mbps
iperf3 = 468Mbps
Minimum idle = 68%
Average idle = 74%
For the final test I selected eth1.10 (wan) from the dropdown menu on the sqm-qos page and retested. This time I maxed out at 280Mbps and thesr are the speeds I should be expecting.
eth1.10 (wan)
Ookla Speedtest = 281Mbps
iperf3 = 447Mbps
Minimum idle = 37%
Average idle = 50%
From the results it seems there is something throttling the WAN - WLAN speeds as iPerf3 is unaffected via LAN - WLAN.
I tried looking at the startup scripts for sqm-qos but it's too complex for me to understand. Obviously selecting pppoe fixes the startup issue but it's a shame I can't get it to behave the same way with whatever interface is selected.