I am a total beginner in this regard, but maybe maybe this could help you:

Check out PBR - policy-based-routing. On OpenWRT there is an app for this task, vpn-policy-routing (together with luci-app-vpn-policy-routing for a GUI inside LuCi). This app helped me to route all traffic coming from my PiVPN to the WAN interface, instead of my commercial NordVPN. Now at home all my traffic goes to NordVPN and when I am not at home I can easily VPN into my home network.

openwrt.org/docs/guide-user/network/routing/pbr

Other than that, the OpenWRT Forum is very helpful, don’t hesitate to ask your questions there.