At first I was getting owned by the enemies constantly. Then I went full into kamikaze mode and that was when the fun started. In general the game feels relatively hard. From what I remember, the enemies need artificial stupidity, not too much, not too little. It's also helpful for creating difficulty levels.
I feel that the damage of the guns varies, My top is:
1. Redemption
2. Hand of God
3. Requiem
4. Lady Mary
All the ideas that I could give you for enemies I'd take them from tibia, so take a look directly at the source:
https://tibia.fandom.com/wiki/Creatures
Take a special look into demons, the undead and the sorcerers (Humans).
I saw that you made a linux build yesterday, so I happily did another try, and look what I found.
You can get stuck into the walls while jumping, also you can move through them.
While paused, you can throw bombs.
The volume settings are loaded only after entering in options.
-edit: you can dash while being in the air, I'm not sure if this is a bug or a feature, but I'm adding it just in case.
Your game made me come back into FPS, so take my comments on difficulty as someone who stopped playing FPS almost a decade ago.
In general I love this game. Keep up the good work.