I'm an FI who's never had aspirations outside of instructing. While that might resonate well with some of the posters here, I'm very aware that FIs like me are limited in some respects by having no line flying or operational experience. Perhaps it's not needed in elementary flying training but some of the ppl students with commercial aspirations would benefit from being around a "proper" pilot. So the "career instructor" is not the panacea of instruction.
And for the OP's benefit...the optimum situation for an FI is to be part-time and have another job to pay the bills. I couldn't do this full-time and pay the bills. I have to say that of all the things I've ever done (BSc, PhD, CPL, IR, and the rest), the FI course has easily been the most satisfying, challenging, gratifying and fun course I've ever done. Just don't rely on the qualification to pay a living wage (I average 15 hours a month as a part-timer).
Good luck!