I think I have a couple of ideas that can help you with the game, they are inspired by Metal Gear Solid 2 that has a scene similar to your game
-In addition to moving crouching you can try to add moving on the floor crawling or dragging to hide behind walls or small objects even enter tight places
-You can try hitting yourself against a wall and moving forward both standing and crouching, this could help the player to move through small places or to be stealthy
-Finally to improve stealth you can add walking and running as in MGS, where walking prevents the NPCs from hearing you nearby but if you run you will alert them by the noise of footsteps
They are just ideas that I had, you can try them if they serve you in any way and I hope I have helped you