I think you can make relatively fast progress to 225-250 watts, you'll be on the elbow of the curve. After getting to that (very solid) plateau, each additional phase of improvement gets harder.
Everything I've read says long pulls at Z2 are critical.
I think you should have a cycling goal along with your FTP goal, something like finishing a regular 20 mile loop in under XX minutes or a serious hill climb goal.
Good luck!