You will need a monitor and mouse initially to install the SD card image of Debian. After that, you can remove the monitor and mouse and use fastboot to flash the console-only “developer” release. Once there, you can use the serial terminal to connect to the board using eg: minicom, or you can ssh in once networking is set up.
While you’d think you should be able to use fastboot to flash the developer release from the get-go, I’ve found that it always fails on a new board until you’ve done the SD install first. It might have to do with partitioning but I’ve never looked into it.