Do you have garden access and do you use the up and over door much?
I have a lockup (as an overflow for my own garage) in an secured area and these used to regularly get broken into, generally by levering the up and over door and folding it over on itself to clear any defender that the person had.
What the owners of the block of garages have done is to hammer a wooden frame inside the metal frame of the door. If the wood fits tightely, and then is screwed into place the door is almost impossible to fold over itself.
If you don't use the up and over door, I'd frame it with wood, and then drill holes in the sides and use 13mm hex head coach bolts into the actual garage door opening.
For your own piece of mind you can't beat having an alarm though that is connected to your house.
A lot of roller doors are surprisingly easy to break into, especially if there is room in the road for them to drive away from the door after hooking something underneath. My parents neighbours had this happen to them at night and nobody heard.