We don't deal with DSC systems so not familiar with all their equipment, however if this was a system we were installing here is what we would do.

Run 2x 22/4 or 18/4 from house to Garage (1 is spare)
- Install local power supply and zone expander in enclosure(s) in garage
- Run all garage devices back to the above enclosure(s)
- When programming, make the garage a second partition and probably not allow for any Stay zones (unless there is a granny flat above the garage). This will allow the home owner to Stay arm the house and away/full arm the garage.
- If home owner wants a siren out there run an 18/2 from the house to power it