Hi Francophile,
I am still an undergraduate, but my focus is on theoretical linguistics and Mandarin. To your most recent inquiry, I can only share the state of my current intentions.
A PhD is a tremendous commitment. It's highly specialized. Finding the right school and program is also a daunting task. I cannot begin to imagine transitioning into a PhD program without having first explored the field further by way of obtaining a master's degree. My plan is to complete my undergraduate thesis and move to Taiwan or China, further explore my interests in the interface between Mandarin and linguistics, and only after having done so, make a calculated step to move forward with a PhD (or otherwise). It may turn out that I find myself more fascinated with the computational or cognitive neurolinguistic conversation. Take your time, but if you want to examine a foreign language of your choice through the theoretical linguistic lense, there are many specialized programs that focus precisely on that purpose.
Hope this helps. I've tried to remain concise and relevant to your question.
Matthew