Suggestions for securing the garage
Slowing the [thieves] down.


I guess this is mostly a matter of fitting better locks, and/or a stronger door. You might be able to reinforce a flimsy door. You might be able to add an extra lock. 

PIR triggered lights may have a slight deterrent effect.

You might contact the crime-prevention officer in your local police force. They may be able to provide advice and guidance.


  Making noise and/or signaling the attempt silently to my phone.


I would consider fitting an alarm. A typical alarm will make a beeping sound when the door is opened. This gives you ten seconds or so go to the control panel inside and enter the disarm code. The warning sound will startle an amateur burglar and they usually won't have time to do anything to disable the alarm before the full alarm sounds.