Yup, only fun part of TW games is the start and building up to a point. Once you can win by pure inertia, not even having to bother to manually fight battles, the game is pretty much over. You can set yourself house rules, I did a R2 game where I wouldn't allow myself more than a certain number of provinces and armies and would build retarded alliances to have completely ahistorical results, like Babylonia restoring an empire on the fertile crescent, Numidians becoming the uncontested power in north Africa, Celtiberians conquering most of Gaul and northern Italy, Epirus conquering mainland Greece and Magna Graecia, stuff like that.
Didn't work very well on previous TW games because diplomacy was useless, but alliances have a tendency to hold pretty well in R2 at least. The hard part was not being able to sell or give away provinces, have to hand-hold AI ally armies and obliterate any sort of resistance so they can move in for the capture.