It's interesting to consider the ultimate lock, but you must also consider the psychology of use by the cyclist, and of the thief.
No matter how heavy your lock is, the consensus of experienced tourers is that you cannot leave the bike, locked, exposed for long.
That is why I'm keen on Dutch-style frame locks, which are already on the bike when you want to go somewhere, weigh little, don't require you to get your hands dirty, and can work with a chain or a cable if you insist on backup.
As described upthread, I use a variant, a steering lock, which makes the bike unrideable and very, very unwieldable, dangerous for anyone smaller than me. it weighs nothing extra, in the sense that the stem and handlebars I removed to fit it weighed more than the secure n'lock stem and handlebars. The handlebars also have a cable built-in if a pole is handy. A longer cable, like the handlebar cable, works with the steering lock on a single key.
The inconvenience of a frame lock or a steering lock is fleeting, like leaving and returning to a car. This can never be true with a monstrous chain: you need to assess how long the monstrous chain will remain a novelty you give the benefit of the doubt. Not to mention that I can think of dozens of places where a good strong length of chain will itself be a target of theft.
If you then add a cheap but loud alarm to draw attention to the thief, you've given him a very solid message, much more impactful than a chain for which he came prepared with an exposure-to-completion time budget and precisely the right tool. Dan pointed out to me years ago, and I haven't forgotten, that I'd rather the thief lost hope and moved on to an easier target, rather than that he damaged the historic, irreplaceable bike I ride before he decided the theft was taking too long.
I have an Abus Granit X D-lock which fits very tightly around my bike's frame, Rohloff wheel and Big Apples, and a lamppost. I don't carry it. I don't need to. I've successfully psychologically monstered the light fingers here that my bike is broken any time they see it standing with the front wheel pointing in one direction and the handlebars in another.
If you do decide on the big chain, I look forward to your reports on its use, including psychological impact on suspicious characters. If you decide on the systemic approach which has worked for so many of us, in all its variations, let us know that too.