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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

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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.