The other issue is in book 3, where the biggest retaliation Xanthir Vang can take is an assault on Drezen, with several armies of demons attacking the city. Even with the defensive bonus from the fortifications, the Sword of Valor weakening the demons, and the Planetar being brought in to protect it, I'm not sure the two armies you have at the end of book 2 will be enough. Even a Small or Tiny force of Drake Riders would be a problem for your forces, for example, and a Medium force could tie up your planetar while other demons take out the human troops.
I'm thinking of letting my players buy Build Points to improve and maintain their military. I'll probably start with 1000gp a build point to represent subsidies from the Royal Treasury and enthusiasm to follow such famous and successful heroes, with the cost increasing as their forces grow larger, and possibly requiring the party to build and protect small settlements that act as trading posts for supplies to reach their armies.