Step by step guide for VPN & split tunneling

Installing and Using OpenWrt Network and Wireless Configuration
4

PBR does not alter the default gateway (it is in paragraph 10 FAQ, but I can understand you do not want to read the whole guide)

For WireGuard you can disable the default gateway in two ways:
Interfaces > WG interface (/etc/config/network)

  1. In the Peers section, disable "Route Allowed IPs" (option route_allowed_ips '0')
    or my favorite way
  2. Under Advanced settings > "Use Default Gateway" > disable/untick
    (`option defaultroute '0' )