[dmjlambert]

A couple of thoughts:

As MagmaJct mentions, the way you have that cut-off switch wired, it just prevents the starter motor from getting power. But that doesn't mean you have to wire it that way. You could go ahead and run the large wire directly to the starter as from the factory, and then run the smaller wire (yellow wire on your truck) through that cut-off switch.

I recommend make sure you have a 16 GA fusible link wire connected to the battery and then connect to the rest of the electrical system from the fusible link. Fusible link is a special type of wire that will burn up before the biggest unfused wire in your electrical system (12 GA) and therefore protect the system, so it is sort of like a fuse. It has a special non-flamable insulation.

I recommend not using repair terminals, the type with a clamp that goes on the battery terminal, and another clamp to connect the wires. Get a fresh wire with the battery post connector built-in.

So, if you want to use that cut-off switch, get a fresh positive battery cable from O'Reilly and connect the fat cable between the battery and starter. I bought the 4 ft 4 GA one for the positive side and the 30 in 2 GA one for the negative side. Connect the small wire leading from it to 6 to 9 inches of 16 GA fusible link. Connect to the other side of the fusible link to enough 12 GA wire to go to the cut-off and back to the junction block on the fender.

My thoughts do not address your key-off drain, but tips about that stuff are covered by others such as B. W.'s answer. The cut-off switch is probably a good addition to your truck.