If they’re just internal the simplest way is to add another IP on the same interface to whatever is serving your service, then bind the service to that IP and add the entry in DNS.

If for some reason you want to keep everything hosted on one IP, for a reverse proxy, caddy is pretty simple. An example caddyfile would be:

http://service1.devicename.lan/ { 
    reverse_proxy 1.2.3.4:9005
}

http://service2.devicename.lan/ { 
    reverse_proxy 127.0.0.1:1234
}

This would also allow you to set https in the future using ACME (dns method if internal only) or your own CA / custom cert.

Does this work different than the policy key? As far as I know the keys under software\policies are used by both group policy and intune, and honored by windows home editions so I’d expect they’d work more reliably than an outlook config setting.