I'm no expert on the prawn, but I believe they don't actually eat the fish poop only pick through it for undigested food. Also, that sounds like it would create a water quality problem for the prawn. I might be mistaken, but I think they are fairly sensitive to the quality of the water.

Since you only want the prawn as fish food for the Tilapia, how about replacing that portion of your concept with Duckweed, Black Soldier Fly Larva, Red Wiggler Worms, and Algae (with all the zooplankton it would attract). This would be easier and give the Tilapia a wide range of diet to keep them healthy and growing.

Also, you can consider just setting up a few ponds with "green water" (pea green soup of algae) and then putting the Tilapia fry in there. Depending on your weather conditions (where do you live?) you can grow out fish very easily and with less maintenance. The only problem with this is you will need a way to control the population after the first season or so because it will get pretty crowded (they reproduce like rabbits). You might be able to just add a few predatory fish to control their population as well. Draining the pond is another option or having a big net sit in the pond is even yet another option.

There are many choices and it's definitely something you can do. You just have to figure out what works best for you and your geographic area and how much time and money you care to put into it.