Probably the router you have doesn't have more processing capability. The task is quite heavy, to do vpn at 300Mbps WAN and SQM. If you don't have a bufferbloat, I think you don't need to run SQM.
You can try upgrade your router with X86 based system, Intel or (mini) pc processor probably fine.
But, before you buy another hardware. You can try install OpenWRT X86 64bit on a USB flash, run it on your laptop (or any other unused laptop if you have one). Config your lan on laptop to connect as WAN interface (connect it to your LAN modem) and you can also use your laptop WIFI as AP. To connect to another router, you can use a USB 3.0 to Gigabit LAN converter as downstream to your local network (to your main wifi router).
Once you set it up, config your VPN and SQM, then check the speedtest again. After the test is done, then you have your option.
Another option is just leave the main router as it is, you may need to setup VPN on every client.
Old discussion here