Deferring start of PhD program because of self-doubts
Yes, it is normal to have self-doubts and imposter syndrome is pretty common. However, the external validation you are getting seems to indicate that your fears are unwarranted. When other professionals and "strong" people consider you a top candidate it is pretty likely that you you are, no matter your doubts. 

However, burnout is a serious problem for some, and finishing one degree with a flourish may leave many exhausted. You should work to avoid this, partly just by thinking of the new program as a fresh environment, but also, by pacing yourself just a little so that you don't overcommit too early. 

Don't worry about job markets six years in advance. It is nearly assured that they won't be the same as now. They may be better or worse, but it is not possible to predict so long in advance. You also have the intervening period to work toward your goals and even to adjust them. Aiming to work in academia or industry, again, need not be a decision that you can't revisit over time. 

You seem to have an opportunity. Better to grasp it, IMO, than to let it pass you by, possibly leaving you regrets in the future. 

I don't advise taking a year off for the reasons you state. If you want to do so to get industry experience or solve a burn-out issue that might be good, but don't just let doubt keep you back. Others believe in you. Believe in yourself. That is doubly true if the position you are now offered won't assuredly be available in a year.