Slabs, stairs, tridents, elytra etc.

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

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

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