Quick dumb questions for a dumb idea.

I would not worry about protecting THE STARTER CIRCUIT, because any thief that knows anything might know how to "jump" the starter.....right at the starter.

So go elsewhere.........

You have a tach? Most people are not gonna re-wire the tach circuit or do a lot of troubleshooting. A simple switch to ground the tach wire (unless it's a CD/ MSD) will stop the coil from firing. If the thief leaves the thing hotwired, this may damage something and might not. Many ignitions conduct current when stopped, anyhow

"Improbable" (not obvious?) interrupt of ignition power. Let's say you have a factory Mopar breakerless ignition. Unwrap the harness and find the power wire right to the ECU. Break that and insert a series switch, make it look "stock."

Fuel. You CAN BUY electric fuel cutoff valves. If you use an electric pump that's even easier, you can put a switch in the pump relay line

There's all kinds of easier ways to protect your car.....than to mess with the solenoid

AND THE PROBLEM IS if they show up with a trailer, wrecker, or roll back, you are screwed unless you have a CELL BASED security device