The setup of the Express as a router is not quite correct and you will get errors in the Express which you can ignore. For independent setup of a device like your mixer it is your choice to configure it on the main network or off it. You appear to have chosen off your main network. There is nothing wrong with this. Most people do find it confusing having different wireless networks, but maintaining a couple of different networks is no problems especially if you make it clear via the SSID used in the router to specify the task.
On the other hand while your device is connected to the Express you don't have internet. So interconnecting the networks is a good idea for most setups where it is convenient.
You did not give us any info on how your main internet is setup or if you want to run them together.
Just to explain a bit further wireless in a client like your mixer usually has two modes of operation.
Infrastructure mode where it joins to the main wireless router. So you don't use the mixer standalone but as part of the network.
Ad hoc mode where it allows clients to join the mixer, but in isolation of other networks.
Your setup with the Express is continuing to use the mixer in ad hoc mode. Simply replacing the internal weak wireless with a better external unit presumeably connected by ethernet to the mixer.
