Change routing priority on iMac
You can set the service order in the Network preference pane in Preferences.app.

Open Network preferences, click the small cogwheel icon below network connection list (next to the + and – icon), choose "Set Service Order…" in the menu.

Then you just drag the entries in the dialog (w/o typing all the interface names).

Btw., in the Network pref pane you can also delete unused network interfaces (– icon) or make them inactive (cogwheel menu).