Personally, I would put mechanical filter media in the first chamber then the bio media in the second. Lots of filter wool should be enough to trap particles. I would also ditch the carbon because it isnt really needed.
With the design of the sump, I think that if you stuff the second chamber with filter media only the upper part will receive water flow.The last baffle should be elevated above the sump bottom and above the water level in order for the water to be channeled through the entire area of the second chamber.
18 gallon low-tech planted community