Any way to change my caller id? Is there any way to change my own caller ID so the person I'm calling sees my Google Voice number instead of the number associated with my SIM card?

My Google Voice number is my primary number and I prefer everyone to use it, so I can receive calls on my PC, while travelling internationally etc.
I think you need to have Google Voice initiate your calls rather than the dialer app.

Generally speaking you'd change the setting in the Voice app for "Making calls" to "Use Google Voice to make all calls". If you'd like to be prompted before the call, then you can use "Ask every time I make a call". (There's also an option to use it just for international calls.)