If anyone stumbles accross this post, here is how I solved it for me:

• PiVPN doesn’t need any special firewall-treatment in OpenWRT, just use port forwarding (Source zone: WAN, Destination: LAN, with the Port you chose, 51820 is the standard for wireguard)

• A special problem I had: I used NordVPN on OpenWRT to tunnel all my traffic through there. When I connected the PiVPN, it didn’t work at first. I had to use something called PBR - Policy-Based-Routing to send all my traffic from the PiVPN to WAN. It was easy with the openWRT-app vpn-policy-routing plus luci-app-vpn-policy-routing