You need to raise the latency of your Aggregate device or else your CPU won't be able to process data in time, and that's the reason for the overloads. Try 1024 frames first, then 2048 if your problem is not resolved. The more tracks you use, the higher the latency should be set to avoid errors.
Also, I'd recommend you to delete those two busses you created. They can be useful if you are applying global send effects to several tracks (i.e., you're using a reverb or delay to simulate an specific audio environment). Otherwise, they're adding an extra stress to your CPU, and you are not improving your sound in any way by using them.