As a workaraound, you could start a timer with the length it usually takes the door to open / close to represent the opening/closing. As long as the timer is running, it is probably in transition. The Chronograph - Adds precise timer, stopwatch and transition functionality to Homey or similar allows for testing if a timer is running, and/or you can change a variable when the timer starts/ends.
If you cannot get the device to show up in Alexa, I would look at the [App] Virtual Devices to find a kind of device that does show up and allows control. If you find one that works, you can use flows to link virtual device state and actions to the Aeotec controller.
That, or you could use favorite flows that Alexa can call to control the garage door.
