Start from external disk only if connected to iMac
Solution 1:
Holding down the option key will involve the Startup Manager. If you hold down the control key before making your selection, then your selection will become the default. This works for both internally and externally installed operating systems.
If Windows on the external drive is the default and the external drive is not connected, then the firmware will search for another operating system to boot. The preference is to boot macOS on an internal drive.