Re: what is the ultimate long range cartridge
If your definition of long range ends at 1000, a .308 is tough to beat as a well rounded option.
It offers the best ballance of availability, known dope, cost, barrel life, terminal effect, etc.
There are more accurate calibers, there are faster, there are harder hitters, etc. But for everything you gain, you also can loose something.
In todays world, 700-1000 really is not even considered an exreme range anymore. It all boils down to what you want to use it for.
Just taking your original question of "what is the ultimate long range cartridge" though, I would have to answer that the 375 CheyTac is the king for now (but that does not take into account the conservative ranges you stated).
375 CT is a contender for reliable 3000+ yards hits.