I'm about to tell my advisor I want to leave my PhD. Do I mention this when scheduling a meeting or do I drop a bombshell while there in person?
If you think that there is any possibility that the advisor can adjust your working arrangement enough to let you continue, then it would be better to give him/her a heads up before the meeting. That way thought can be given before the meeting and so it needn't be so ad-hoc. 

But have an idea of what you would need for yourself going in to such a meeting so you have some basis for discussing options. 

Leaving a degree is a big step, make sure it is the right one. And make sure that others have a chance to try to work it out with you if there is any way possible. But for that, they, themselves, need a time for reflection. 

And, as per your final thoughts in the question, have an idea about what you want the alternative to be. Ending with nothing isn't a very happy outcome. 

But the goal isn't to get you to change your mind. Rather it is to find a way so that the conditions are appropriate and the future is clearer.