Customized truck alarm system

Hey all, I'm in the process of learning/researching/designing an alarm system for my truck. I've never done any arduino projects but I've got a little bit of coding experience and I've done quite a bit of learning recently. My design involves triggering an alarm siren and a text message sent to myself if any of my sensors trip: 2 PIR under the truck to detect someone stealing the catalytic converter, 4 magnetic door sensors in the front cab for each door, and 1 magnetic door sensor in the back camper shell. I would like to power the arduino, PIR sensors, and GSM module with my 12v lithium ion goal zero battery which is in the back of the truck. I'm planning on mounting the siren and arduino in the engine bay.

I was planning on using a remote controlled switch to turn on and off the entire system since simply turning off the arduino would still allow the other devices and sensors to draw power from my battery. I usually live in my truck and sleep in the back so this would be something to protect nearly everything I own and is important to me.

I've got some questions and I would like some advice and critique from anyone willing and knowledgeable. Here's some things I'm not sure about:

-Falsely triggering or not picking up signal from my sensors since the wire will be run anywhere from 1 meter to 3. They will travel on the same path as my 12v power cables and run underneath the truck along the frame. Should I use a certain type of wire gauge and insulation. Should I be using resistors or capacitators?

-Will drawing 12v power off of multiple electronics require anything besides the 2 buck converters I have in the diagram?

-Whats the best option for GSM module? With cost of SIM card, text only need, and cell coverage in mind.

You can add a tilt sensor incase someone jacks it up or tries to tow it.

good thinking!!