Very nice! I think it makes a lot of sense to put a substrate around the tub that is unlikely to get dragged or tracked into the tub to add to the sediment and muck that will be unavoidable with ducks. There is a plastic lint filter available for utility sink drains that will stop larger crud. They are about eight inches tall, so water can go down through the open top, but stuff that has sunk to the bottom has to pass through the plastic blades of the filter. It might help keep your drain and hose clearer.
Consider adding an inline valve or even a hose-end sprayer with a shutoff valve to keep your tub from draining. Your most likely issue is going to be the amount of muck that ducks produce in water. So a hose sprayer would probably get clogged. But you can put a quick-connect fitting on the end that would allow you to attach (and detach) either a capped-off fitting or a hose sprayer or other accessory. By installing a cap, you would be able to release the muck first before attaching a sprayer or other accessory so they wouldn’t get clogged.
We change our duck pool water every day. The muck settles to the bottom and does not stay evenly suspended in the water. It settles as an inch or two of sludge at the bottom, which is part poop and part dropped feed. Feathers and oil are in there, too. Our ducks have other fresh water buckets, but once there is clean water in the pool, that’s where they want to drink—and drop feed out of their bills. I have to stir up the muck in order to get it to dump out of the pool with the water. Even then, I have to rinse it out.
Whatever hardscape you install, I would encourage you to do so in a way that makes it easy for you to get access to your whole drainage system.