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I recently setup OpenVPN with selective routing and I used the rules from the WireGuard Selective Routing to External VPN Endpoint opnsense guide. https://docs.opnsense.org/manual/how-tos/wireguard-selective-routing.html
I did have to setup a port forward for ProtonVPN dns server and the and the lan rule I kept destination as any instead of the rfc1918 alias.

Disable routes checked on local config make sure you have dns set in system setting general and make sure you're getting a handshake on Wireguard

Are you changing the listen port example first Wireguard vpn listen port 51820 2nd 51821 etc.

I'm no expert so someone with more experience will come along on that note my solution.

Firewall-Nat-portfoward click add
Interface lan tcp/iPad version ipv4
Protocol tcp/udp
Source you're alias for routing over vpn
Source port range from any to any
Destination any
Destination port range from dns to dns
Redirect target ip single host or network put you're vpn providers dns or any other dns directly below single host or network
Redirect target port select dns

That should be it save and apply.