I'd agree with just doing the ATPL studying. The CPL studying is almost as tough so why not do the ATPLs from the start ?
I think there are both cheerful and not so cheerful instructors out there. There's no denying that the pay as a PPL instructor can make for tough living. But the flying is fun, you get to help other people learn to fly and meet a lot of interesting people (students and instructors) along the way. You'll build hours and once you have enough experience you can make the decision to either try for the airline / bizjet route or instruct for the commercial licenses.
Good luck with the career change.