There seems to be two ways to deal with poop on poop boards. Some scoop/sift while others slide them out and dump them. I'm a scooper/sifter. Looking at your coop, you won't be able to get in to scoop, so if you use a poop board it will probably need to be the kind you slide out. Whether you then scoop or dump the contents is personal preference. Multiple small trays would be a pain but one long one might work best.
Your roost is low so realize that whatever bedding you use will get kicked onto your board. If you plan on using two different substrates (one for bedding and another for the board) they will end up getting mixed by the birds scratching around.
If you keep bantams you can get away with a narrower board, but standard large fowl need a big board. My board is 24" from front to back with the roost centered over it and my birds still manage to hit the edge every once in a while.
If you use pdz or sand on the board the birds might decide to dust bathe in it. I've found that 1" of pdz is fine.
Given how your coop is configured and how low your roost is, I'd be inclined to use shavings in the whole thing. I'd put a poop tray under the roost that can pull out and be dumped every couple of days, daily if the birds are spending lots of time in the coop or if the coop has less than ideal ventilation. Stray poops will get worked into the shavings and you can do a whole coop clean seasonally or as needed.
Whatever you do, you're going to want to install a lip on the edge by the big clean out door so when you open it up all your bedding doesn't fall out.