How to use WiFi for everything, except a specific connection to my NAS (Ethernet)
It dumbly uses whatever is in the service order, whether it's Ethernet or WiFi. I had hoped it would choose the best route.


It doesn't "dumbly" do anything.  It performs as expected; if priority is given to WiFi, it will direct all traffic over that connection.  Keep in mind, macOS (out of the box) doesn't prioritize traffic as it's not a router/switch.


  Question: How do I have both adapters enabled, prioritising WiFi, but
  utilising Ethernet for my NAS connection?


Put your NAS on a different subnet.  You can do this with a different switch, with VLANs (if your switch supports it) or you can direct connect it.  Just keep in mind if you want to share access to the NAS, you either need to have a multi-homed (multiple network adapters) on your NAS or you need a router between the networks (a layer 3 switch would be nice, but they're expensive)