To solve the problem of too much monster spawn, why not have lairs that players can go to for a tough(sliding up scale depending on game progression) fight to stop a type of monster spawning in an area for a while. Eventually a new lair can pop up and start generating monsters. Also making the spawning of a lair low, and the spawning of the monster from the lair higher. Maybe a lair every 15 turns and a monster from the lair every 5? Or 10 and 20? Also an alert to the lair or spotting of monsters in the area would be good.