LOL...I think you might be over thinking all this. I live in a dry climate. So any bedding works for me. But if you have a very wet humid climate, you are going to have to concentrate on dry drier and dryest. Bacteria, virus's and worms love to breed in warm humid wet environments. That is why I suggested sand. Sand is dry, even when wet, if that makes sense. LOL You will need some sort of bedding in your coop to help to absorb poop. You can't have a bare flooring. Not sanitary. I am not familiar with poop boards. I tried one once and found my birds wandering around in it the next morning and decided I had had enough of that! LOL So it is up to you.
Birds rarely fall off the roost bar, but it can happen. I would worry about that. But make sure the poop hammock or board does not interfere with getting on and off the roost bar, or they have to step in poop to get up or down.
Gravel is going to be too hard on their soft foot pads, even pea gravel will get stuck between the toes. So your options are sand, wood shavings, sand, grass hay, sand, oat straw, sand, LOL...LOL Sorry.
Here is a link to some bedding options for your coop. I haven't read thru this thread, but maybe you can glean something from it and get more ideas on what you need for your situation....
https://www.backyardchickens.com/t/857605/i-need-your-opinion-for-bedding-please
If you have any more questions, feel free to ask!
