Disconnecting the battery seems like a good way to keep someone from starting it (unless they have the time to re-connect it). Unfortunately, it also disables the factory alarm, which may or may not be effective. It's also a huge inconvenience, since you would need to use the key and pop the hood every time you start the car. With proximity keys, my biggest fear is someone using a repeater to unlock my vehicle, or someone just breaking a window and taking what's inside.
Few Ideas I've thought of, and some I actually use:
1. GPS tracker. I use an Invoxia GPS in each of my cars/trailers. I pay a 3yr subscription on each vehicle. It's as big as a pack of Wrigley's gum, powered by USB and have a 2-4wk battery, draw almost no power. They are wired to an always-on source buried deep inside the interior panels, my trailer ones are solar powered. I can track all my vehicles at all times and you'll never find it, you'd have to unbolt my vehicles/trailers piece-by-piece to get to them.
2. Hidden Fuel or ignition kill switch. You could turn it off each night or each time you park in public. Your keys will still work, it's not super inconvenient, your truck will simply not start.
3. Mount insanely loud sirens and red/blue flashing LEDs all over your vehicle and wire them to your dome light. They would only come on when the dome light comes on, but there will be a bypass switch on the outside that only you know about. Arm the car by turning on the switch when you leave, disarm the system before unlocking the door.
4. Same idea as above, but use an in-cab motion/occupancy sensor to detect someone entering the vehicle, breaking the window, or reaching in an open window. Ultrasonic sensors are great for this purpose and can be hidden in the pillars.
5. At home, if you don't have a garage, try parking in the side-yard or behind a fence. Create a desire path where a person will be funneled through a gate or opening. Using a decent IR beam sensor, mounted at the correct height, you can trigger lights and noise to scare someone off. Better yet, have it kill ALL the lighting, then have a speaker play the sound of slow footsteps in the leaves. Get creative with the special effects. Maybe some bright strobes and loud booms to simulate the sound of a flash grenade.