This is kinda smart, maybe, but it has a downside.

If a sufficiently advanced AI , in the pursuit of completion of its task, managed to ascertain that the desire to unexist was “artificially contrived” it could interpret that as harm, and that might not be good

Hm, if you look at corporation law and accounting, the actual goal of societies (sets of self-sustaining constitutional rules, policies and procedures) seems to be more that of long term sustainability (and even growth), rather than a fixed purpose, lifespan and death. I mean the mechanisms for determining a corporation with a fixed life are there, (and in China they are mandatory, although perhaps de facto permanent with 999 year contracts), but in practice, it's almost always permanent durations.