Hi @Noah_Weiner,
You should be able to see all of the in use IP addresses in your router settings. So you should be able to use the to know for sure if the address is available.
The goal is to get everything involved with Roon on the same subnet. Do you have anything else running on the 10.0.0 gateway? If not it should be safe to switch it. Is your QNAP getting its IP address from the Eero or the xfinity?