You can't create solid custom slabs, stairs and solid placeable items other than blocks. Problem is that Minecraft would show them as transparent blocks because of this limitation: https://bugs.mojang.com/browse/MC-54254
You'll be able to create custom shaped blocks when Mojang will add this feature: a special tag that can be turned on or off, now it's always on. "Opaque".
Another reason is that I can't change hitbox of blocks.. sadly another Minecraft limitation.
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. Read more here.
Minecraft doesn't allow you in any way to add custom textures / custom 3D models to elytras.