Can the Iron Throne legitimately establish a permanent army of the Crown?
We see plenty of examples of Targaryen kings altering laws, issuing decrees, or arbitrating disputes (Jaehaerys I’s reforms, the codification of the Old and New laws, etc.), but it got me thinking about systemic state-building. From purely the book (Fire & Blood), we can estimate Kings like Jaehaerys I Conciliator (and his wife Alysanne) or Viserys I have made enormous changed in the Realm. Jaehaerys, for instance, had expanded the roads, connected the Seven Kingdoms via roads, and even disarmed the Faith over the Crown.
So, I think we can assume that the King can do whatever he wants with the laws and within the Crownlands. However, what if a Targaryen king, or say, Robert (due to his extreme fear of Dany's pregnancy and Khal Drogo in AGOT) wanted to create a permanent army of the Crown that's consisting of hundreds of men (who would be recruited from the slums of Flea Bottom or Crownlands), be trained by legends like Barristan Selmy or Jaime, and be armed with weapons like polearms, swords, shields and bore the banner of House Baratheon and Lannister? Could it work? How would this institution fare during the War of the Five Kings, or Blackfyre Rebellions or Robert's Rebellion (if it were founded before Robert)?
Bonus: How would your army of the Crown be in Westeros?