Regarding network access requirements for a NAT Type of Open, Microsoft's Xbox One NAT Error troubleshooter says this:
If you're connected to a network through your workplace or school network, ask the network administrator to open the following ports:
- Port 88 (UDP)
- Port 3074 (UDP and TCP)
- Port 53 (UDP and TCP)
- Port 80 (TCP)
- Port 500 (UDP)
- Port 3544 (UDP)
- Port UDP 4500 (UDP)
If you’re connected to a home network, you may need to open ports on your network hardware yourself through port forwarding.
The list above makes some sense for users requesting access from a network admin, e.g.: on a school or work network. It's fairly possible that such institutional networks may block traffic in both directions on some of these ports.
However, on your home network, it's most likely that all outbound traffic is being allowed by default. Therefore, you should only need to do port forwarding for inbound traffic.
Some of the ports listed refer to services that I don't expect should be running on an Xbox normally. (53 and 80 especially - DNS and HTTP, respectively) Thus, the Xbox network traffic addressing those ports should all be outbound and therefore not require port forwarding for a home network.
I don't want to forward any more ports than my Xbox actually needs. So, which of these ports are actually receiving inbound traffic and essential to achieve an Open NAT Type?