You have discussed before some alternate sources of power the Targaryen’s developed such as public works, religion, public relations, military campaigns and bureaucratic competence. What other sources could the kings have turned to once the dragons died (ie. agriculture, finance/trade, etc.)?
What could be done to encourage nationalism in Westeros as it was when the series started (ie royal progresses -> royal marriages)?
Well, you’ve mentioned most of the big ones. Agriculture I don’t really see working, outside of a more general application of Aegon V’s relief efforts. I suppose you could try a Chinese or Japanese-style Imperial land reform, but that would lead to a quick coup by the entire noble class.
Finance, I could see a bit more of a role. A royal bank with branches in all of the major cities of Westeros could be used to build a direct relationship between the monarchy and the burgeoning merchant class that could eventually be used to bring the aristocracy to heel in the name of a unitary nation-state. (For more in that vein, a big part of why I’m so big on city charters is that creating civic governments is a way of uniting the merchant class as a self-aware politically active class, that just happens to have its own armed forces through the city militias)
But the main neglected thing, as I’ve said for a long time, is the justice system. No matter where you look in Medieval Europe, one of the most successful strategies for expanding the strength of the monarchy is creating a royal judicial system that can hold the nobility to account for their actions against the peasantry. Not only does this create a comprehensive bureaucracy of judges and sheriffs and the like who embody the nation-state and a sense of having rights as members of the nation-state, but it gives another way to attack the power of the nobility.