What are the disadvantages and advantages for a senior graduate student to restart after 6 years' study?
It is very common to be beset by self-doubt at this point -- just when you are poised to begin writing.  (It's best if you can write as you go along, but sometimes the nature of the project does not permit that....)

(There are questions with good answers on this site about how to keep up your nerve during this final stage.  Helpful reading!)

In your case, these self-doubts are complicated by some weaknesses in your department and some frustrations you've had along the way.

Let me draw an analogy from parenting.  Most of the time, a parent will do best if he is really tuned into his baby or his small child, and his antennae are set to pick up subtle vibes from the child, so as to be as responsive as possible.

But there are situations in which this is actually counter-productive, and you have to detach yourself, and care for the child more or less on auto-pilot.  For example, if your child is the non-responsive type, and you are getting frustrated.  In this case, you have to back off emotionally, so as not to overwhelm the child.  It's okay, it won't be forever.  You just need to let the child's brain mature a bit more, and then try again, in small steps.

In your case, the thing you're going to have to train yourself to ignore, on a temporary basis, is the niggling voice in your head that makes the negative remarks and the probing, self-doubting questions.

You are very close, just keep plugging away!  Crank out an imperfect dissertation and move on with life.  You are not the only talented person to have attended a school that has some weaknesses.  Think of the Third World!  I once attended an agronomy class, "Farm Machinery", that was taught with posters the instructor had made on his kitchen table.  An actual tractor was not brought in until the last day of class, and then we did not even practice changing the oil.

For your postdoc, forget about rank -- look for a position where you will have comfortable, fruitful collaborations.