Okay, my 0.02E worth.
On W&B: Have you ever (I mean ever) flown with an aircraft even near MTOW? During my PPL training I once had a student in the back seat to observe, which still left us maybe 60 kg below MTOW. That was it. All other flights were two-up, at least 100 kg below MTOW. At MTOW, or even above, aircraft really do fly differently, especially if you apply the same Vref and other speeds, and the flare pitch-up indifferently.
Nobody here will actively encourage you to depart above MTOW (although some of us may have done so, and lived to tell the tale) but even flying right at MTOW is something that you will want to experience first, when the other aspects of the flight are well within your personal limits and experience.
Next, passengers. Have you ever flown with passengers who have no experience in light aircraft? You will find they take up a lot of your mental capacity, because they start to babble at inopportune times, will distract you from your work of flying the aircraft, might get airsick, do stupid things, panic and so forth. Flying passengers in comfort is something you need to learn about, and you will want to build up that experience slowly. Take one passenger up initially. If that works, take two.
(Oh, and by the way, have you ever sat in the back of a C172 or PA28 with another person next to you, for half an hour or so? They are a lot smaller than your average family car, particularly in the back.)
Next, international flight. All of a sudden you have a lot more on your mind. GAR forms, customs, flight plans, different airspace rules, different ways of getting and paying for fuel, different ATC services, you name it.
Next, cross channel flying. Dealing with haze which reduces perfect VFR to IMC conditions anyway. And you have the ditching risk to worry about. Lifejackets, raft, PLB, ...
All this is stuff that can be experienced and learned, no question about that. The question is: Is just a few hours after your PPL exam a good time to learn and experience it all at once? Or would it be better to build up to this experience and knowledge slowly?
a 250 hour fresh out of training atpl cadet possibly at the controls of a A340 mid Atlantic with 200 people on board whilst the captain is having his nap??
I think you'll find it doesn't work that way. On flights where the captain needs a nap, you'll get a "heavy" crew, where at least one of the P2s is a "relief captain": Allowed to act as captain during the cruise. That designation only comes after a significant hours of experience on the line.