I don't know if it was this video or one very similar, but it greatly improved my squad's ability to do harder missions. Knowing what causes more enemies to spawn is super helpful. Key points that suck with me:

• Completing the main objectives are the biggest drivers to increased enemies. After the last main objective is completed, you get something like 4x the amount of enemy patrols spawning.
• Being close to objectives, bug holes, bot factories, and the extact point spawn in enemy patrols. Completing the objective or destroying the hole/factory makes it stop spawning patrols.
• Closing holes/factories and completing side objectives gradually ramps up the number of enemy patrols, which is totally counter intuitive to me. You can destroy like half the holes/factories before it starts making a difference though and even then it's a subtle difference compared to completing the main objective.
• Finally, what this post mentions, is that each "group" of Helldivers increases enemy patrols. So if you split up into 2 groups of 2, you'll get 2x the number of patrols.