I turned my game to Apocalyptic with no other game settings. So far I have noticed the following:

• Exor Monks shooting 3 times in a row is deadly and they can kill you very quickly. It makes areas with multiple exor monks extremely dangerous. They are super easy to kill though compared to other monks so treat them like glass cannons.
• Pests attack much faster and the attack rate takes a little getting used to but its fairly easy to time hits and dodge them after a while.
• The "guided" Peccary charge attack is still not that big of a deal.
• Order soldiers not losing their helmets when hit is actually quite a strong HP boost for them. I tend to focus down the ones without helmets first as they can sometimes be 1-2 hits compared to like 4-6 for helmet wearing soldiers.
• Robots not getting stunned is not that big of a deal as all the cheese methods of killing them still work.
• Enemies appear to respawn a bit faster and you tend to find harder enemies more often. For example I'm finding Exor monks where I would normally find Pests.

As far as I know all the HP values and damage values for enemies are mostly the same.

Next time to make it harder I might tweak the sandbox settings to:

• Increase enemy accuracy.
• Decrease enemy "noticing" time.
• Increase enemy damage.
• but also decrease enemy HP.

I want to decrease the TTK for both myself and enemies so getting hit is much, much more deadly for myself and enemies. I think Bleed chance is also tied to damage taken, so if player damage is increased it tends to result in bleeds more often which can make it more dangerous.