Create slabs, stairs, tridents, armors etc.

Slabs and stairs

You can't create solid custom slabs and stairs. The problem is that Minecraft doesn't allow custom hitboxes for custom blocks.

Tridents

You can't create custom model for thrown tridents as it's a Minecraft bug I can't fix: https://bugs.mojang.com/browse/MC-155286

Armors

Minecraft 1.16 (and lower) doesn't allow you to add custom textures / custom 3D models to armors in any way (you can but only with Optifine). You can make them colored but you cannot change the texture.

Custom textured armors 1.17

To create custom textured armors in 1.17 you can follow the tutorial here (No Optifine).

Custom textured armors 1.16 (and lower)

ItemsAdder can automatically add custom armors to 1.16 version and lower, but only if the user has Optifine installed. Just enable it in config.yml

generate-custom-armors-textures:
  optifine: true

Elytra

Minecraft doesn't allow you in any way to add custom textures or custom 3D models to elytras.