I think what you propose isn't going to work when your openwrt router is running relay configuration and you also want openvpn client on same device. There was a similar post from another member on this forum a few weeks ago. Devices which want to use the vpn tunnel must be on a different subnet for success.
https://forum.openwrt.org/t/multiple-problems-with-openvpn/49041
You can check your openvpn client router is functioning in normal router mode when ethernet wired to your main router. See my PDF for helpful advice.
https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=279
Maximum openvpn speed using C50 v3 or v4 is 13 Mbps over ethernet btw.