It's always a bit of a lottery deciding when to leave the RAF. The airline business is either in boom or bust mode, and I'm not quite sure which mode it is in at the moment. Having said that, ex-RAF chaps of your age are getting jobs as there are some airlines that value experience rather than just taking cadets. You may consider yourself old, but in most airlines (i.e. non BA) your flying hours will mean that you will reach the hours threshold for being considered for command far sooner than the cadets - assuming the market stays in boom mode. If I'd taken the PC rather than leaving after 12 years, the job market and my career progression would have been totally different, and I would have been in the RHS for 10+ years. The grass isn't always greener, so keep on researching.
I'm sure there must be some 'well-trodden path' to an ATPL from the E-3D, it was certainly so on VC10/Tristar/Voyager - ATPL notes etc were 'recycled' between mates and 'top tips' passed on from one fresh ATPL holder to the next candidate. Or is it considered a faux pas to discuss this these days?
As a counterpoint, a contemporary of mine (decorated FJ) decided not to continue with flying and is now a manager in a global company. He gets to see his family a lot.
Good luck with whatever you decide.