This is a decent write-up. Thank you for sharing.
When opening up the existing wire loom, there are tidier ways to open it, but the key is to not cut factory wiring in the process. The wiring in the photos looks undamaged, which is good.
When running the new toggle switch wiring, it's good form to follow factory wiring. If you'd like to have factory-looking wiring that doesn't' require the plastic loom, you can use CL2 or CL3 jacked speaker wiring.
https://www.amazon.com/Outdoor-Speak...01N3JQU7Q?th=1
Using a silver toggle swtich is pretty obvious and easy to see. A black rocker switch is easier to conceal. Avoid locations where thieves would look, such as under the dash or in your center console. Also, make sure it's not a location that could be bumped while driving since it will kill fuel when off.
I had an immobilizer kill switch on my previous car (installed after it was stolen then recovered), going on 8 years now. You have to get into the habit of always using it, and you have to sacrifice remote start. Make sure the on vs off orientation is natural.
I wouldn't be against adding an LED light to indicate status of the switch (DPST) so you'd see the light when going to push the start button. That would be a final check for you to know that it has power before trying to start it, and it wouldn't mean anything to a thief. Ideally, the push button itself would be illuminated and not a random, extra LED.
https://www.amazon.com/uxcell-Rocker.../dp/B07T2RB1TD
the reason for DPST is you'd have one terminal pair for fuel relay control and the second terminal pair for the LED indication - simply 12V constant to the LED. This would also be a reminder to always turn off the switch since you'd then see the LED go out.