Slabs, stairs, tridents, elytra 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 which I can't fix: https://bugs.mojang.com/browse/MC-155286

Elytra

Will be added in future ItemsAdder versions, but will require 1.21.2 clients.