I did another factory reset of the router and the VPN issue seems to have finally been resolved. I'm getting about half my total download speed, but it stays consistent and I know there's always a slowdown when using VPN.
Now I'm experiencing another issue where the router isn't getting the same speeds as my modem. Doing a speedtest on the DumaOS page gives me ~800mbps. If I speed test from my modem, I get 1Gbps. Upload speed again seems unaffected. To clarify (since my issue has now changed) this is without any VPN active, directly connected via Ethernet, measured from the router/modem themselves.