I cant really see how the water flows through the sump but from what I see I would forego the sand and use live rock rubble. Those bioballs collect a lot of gunk and generate Nitrates. You would need to take them out regularly and clean them to keep the 'trates under control. You are correct that the sponge filter with sand on it would slow down the flow of water and some would probably sift through until the sponge became clogged. Sand needs to be on the bottom if you use it.
You might consider that a protein skimmer would help with cleaning the water as well. Mangroves work well, but they can grow quickly and I really cant see how you could use them with this type of sump. I would stick with chaeto or some other type of macroalgae and use a plant bulb for lighting. A filter sock over the input to the sump would catch the majority of the solids from the tank. You just have to clean it occasionally.
You can keep a cleanup crew in with the live rock to help keep the detritus to a minimum. A couple of cleaner shrimp or crabs would be fine. I wouldnt put a starfish in the sump. You can, of course, assuming they can get enough to eat but I would rather see those in the DT.
When filling your sump I would advise that you only fill it to the level where the DT would siphon if you lost power. What I mean is that if your power goes out and the pump stops running, some water will siphon back to the sump plus probably some of the water in the overflow as well. What I did was fill my tank until water started flowing into the sump then, with everything off, fill the sump leaving about 1 inch from the top. Then I turned on the pump and marked where the water level stabilized. That became my max fill line. Then I started taking out water until the pump started sucking air. I put enough back in to raise it 1 inch and marked that as my minimum fill line. (A good auto top-off system will keep the water level where you want it.)
This way if I were to lose power I dont end up with soaked carpets. You can put anti-siphon valves in the water lines but I wouldnt depend on them since they can fail from things in the water that could cause the ball to be stuck open. They could corrode or have some piece of detritus jam it open. You would never know about it until something bad happened.
Another option to add a 'fuge would be to change your sump to one like Kio shows in his pics. All you need is a regular tank, get some baffles made from plexiglass or glass and build it to what you want. Just be sure to do your research if you want to put in a protein skimmer so it will have adequate room to function. There are plenty of videos online showing how to build one.
Keep us advised on your progress.
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....