I'd always wire one 8 cored cable to each detector, even if you are only using 4 cores.
Screened cable is not required for anything on Texecom and yes IMO it is top quality stuff.
Smokes could be wired together on Latch but TBH it's not worth messing about like that.
Stick with one zone, one device, one cable.
It seems extra keypads have been mentioned I would say one upstairs on the landing will be handy.
It means in the night if the alarm activates you can locate the room before storming down the stairs.
One in the garage maybe pointless if it's not large enough for a car.
Maybe you could install the Hall keypad on the Hall/Garage wall then you only have to drill through to install a keypad in the future.
Remember any final exit door should have a contact on it to trigger the Final Exit.
Obviously cables installed to places for future expansion is handy but it depends how much protection you need.
Have you thought about a fixed PA button in the master bedroom or adj front door?