In that case I wouldn't use OpenWrt, which is specifically meant for headless machines.
I would use actual Raspberry Pi OS, which is based on Debian, so you should better search for a guide on how to set up a Debian router.
My quick googling brought me here, although I haven't really read it: https://gridscale.io/en/community/tutorials/debian-router-gateway/
I just searched for "linux router tutorial"
This tutorial seems decent: https://gridscale.io/en/community/tutorials/debian-router-gateway/
Ignore steps 2 and 7.
In your case eth0 is your wireguard-interface and eth1 is your local-network-interface.
Setup the firewall (Step 4) to your liking. You'll probably at least need the following if you dont want to setup proper back-routing for wireguard-peers:
*nat -A POSTROUTING -o eth1 -j MASQUERADE COMMIT
-
After that you'll need to setup your local-clients to use that gateway (e.g. as default-gateway or only the wireguard-network).
As i said, many routers support redirecting, so you dont have to setup routing on every client.