Aiptasia in the weir is good as they are light loving filter feeders, not sure about cryptic zone Loz I would worry that they would try and find light somewhere that they are not wanted, I expect dusters and sponges mainly.
I looked into oysters some years ago, at the time hobbyists were struggling to keep them alive for any length of time, I havent checked back in on the current thinking.
The thing about nutrient export is that you need the nutrients there to export in the first place. With a large DSB, Cryptic zone, rdsb, algae refugium and mangroves I am going to have to tread carefully to ensure that none of these areas are starved. I expect to add to them as the tank grows and fish are added. Whilst I will be setting up the system to have tanks to cater for these forms of filtration I doubt I will start running the tank with them all fully functional, for example there wont be much point having a 4ft tank with 200 mangroves in, an rdsb and 40ft dsb when I only have 4 fish. One of the most infuriating and yet great things about this hobby is that it is always a work in progress.