Appreciate the feedback! The reinforcement period is different for each mission. In mission 5, the first wave takes a while to spawn, but subsequent waves spawn much faster. It's pretty punishing, I admit.
For harder missions, I always pack the Radio Jammer, which reduces the number of guards spawning, and increases the time it takes them to spawn. Possibly one of the more useful late-game items. Almost essential on missions 5 and onward. I'm impressed you got through mission 8, though!
This was a late reply, but thanks for taking the time to play and give feedback. Much appreciated!