What would be the wiring diagram to add a receptacle switch combo to the outdoor garage lights circuit?
yes, your diagram nails it, and it's obvious you have a good grasp of what needs to be done.  

Those "handy-boxes" are great for plain receptacles.   However, GFCIs are too bulky for them.  

For GFCIs, I like to use a 4x4 steel square box, with a 1-gang "mud ring" that sticks out about 1/2", that gives plenty of room for the GFCI and nuts.  

Since what is downline of the GFCI will be lights only, those don't need to be on GFCI protection.  As such, do not use the LOAD terminals at all - leave the warning tape on them. Make all your connections to LINE. 

Another option is to remove the GFCI from the garage altogether and put it in the house. Where? At the switch.  By using a GFCI switch!  Remember, once you complete your plan, you will almost never throw that inside switch.  You are actually better off leaving the PIR powered up 24x7, and you'll be controlling the garage lights from the switch inside the garage.  That's a perfect application for a GFCI switch, which is basically a GFCI deadfront that is rated for switching duty.  The "Test" button becomes the Off switch, and the "Reset" button becomes the "on" switch.  This will protect the entire circuit, lights and all, which is unnecessary, but it allows you to put the GFCI in a box that has room for it, and that lets you use a simple Handy-Box in the garage.