Disagree unless the target nation simply lacks air or navy force, at which point Immense Triumphs are automatic. It would however force you to manage the forces you send in, such that you would be going for less than Immense Triumphs.
FYI: From the perspective of realism, a nation being relentless bombed and its cities reduced to ash or starved into submission via a blockade with its port cities being destroyed would surrender fairly quickly. If that doesn't fit your gaming objectives, sorry.
However, I do like the idea that victory in a war would result in some land going to the victorious nation from the defeated nation. The mechanic could be set up such more land would go to the victor in the case of a victory through ground Immense Triumphs, since--realistically speaking--the victorious nation will have occupied a significant amount of land and or taken strategic objectives inside the nation.
See above. Also, it would mean that everyone would need to have better balanced militaries. Currently it's possible to build up a large tank force and simply blitz opponents who may totally outclass you in every other measure. It's a bit of a loophole that hopefully this will address.