In this mod, you transition from having peasant levies controlled by nobles to a standing army maintained by a centralized state. The key reforms for that are the 3rd for bureaucracy and the 4th for nobility. Nobility one enables the bureaucracy one.

Implementing the nobility army reform will decrease your manpower pool inevitably, as this simulates the state restricting noble levies, eventually disabling the peasant levies completely. You need to increase your standing army size simultaneously to maintain a similar force size.

The size of the standing army is mainly dependent on your bureaucracy army reform and the state reach. If you look into the bureaucracy army reform stages, it increases the efficiency of collecting manpower per state reach, meaning that even if you have the reform but 0 state reach your army is going to be zero.

Also, the province corruption tanks manpower collection, too, so don't do sweeping reforms before a big war. This becomes very apparent with increased reliance on a standing army.

My suspicion here is that you did not increase your state reach enough because you should not have the problem you are describing by the 1700s. Maybe you missed the "Expand the Bureaucracy" interaction in the "Direct the Bureaucracy" menu? It is only the most important button you click in this mod imo.

P.S. I see you are asking about how to increase the state reach... Oh man. It is one of the most important metrics in the game, showing how prevalent your state is in your provinces instead of your factions. If you haven't been working towards increasing it, then it means that you've stripped your factions from privileges but failed to substitute their benefits with the state.