Hi @WilliamT and welcome back to the Forum.
In macOS (CoreAudio Device Settings dialogue in Dorico) you can try to uncheck Automatically Configure Devices: if this option is checked, the Mac will change the output to Headphones if you insert your headphones (or external mixer) in the jack connector, and will switch to built-in speakers if you disconnect the jack. Maybe unchecking this option, and selecting your desired output, can help with your issue? (I also uncheck the microphone input, as in Dorico is not use for it.)
You find the dialogue in menu Edit → Device Setup-> Device Control Panel…
Another suggestion, (with the option unchecked, and an output chosen) is to use Loopback, a fantastic software to re-route (and selectively mute/unmute) your audio signal from any app to any output (is faster that entering the CoreAudio Device settings)