For DMZ you need to put the WAN IP adress of your XR500 in your Fritzbox. Or preferrably the MAC adress so it's always routed right. I looked up a manual for Fritzbox 7590 though and I'm not sure this router provides this option.
Then the other way is to forward TCP / UDP 1-65535 to your XR500.
On XR500, you don't need a static route for a static IP on your PC, you can make a IP reservation under XR500 LAN settings. But really you don't need this either since uPNP is supposed to take care of this, doesn't matter what IP your PC has.
You're using uPNP and port forwarding at the same time, best is to let uPNP do it's job, or if you want the manual rules turn off uPNP.
The issues with PPPoE / VLAN only affect your upload QoS by the way. If you have enough bandwidth I doubt it makes a difference.
DNS servers don't make a difference for your in game connection quality, usually I just set them to ISP native DNS servers.