If you want to use nat reflection, just set that up on your nat rule on your wan that redirects 9000 to 3389.
If your going to allow rdp from public.. Agree horrible IDEA... and you don't want to remember to use :9000 or not... You could change your rdp to actually listen on 9000 then just port 9000 to 9000
And setup a host override for name.tld that your using to access your public IP outside, to point to the rfc1918 address of the rdp box when your internal.
You should really use VPN to access your rdp resources while your outside your network