also, have you done all the mundane mac crap? login items, networks, etc? that may be all you need to do. i have a 2012 macbook pro and i can run pretty large projects at reasonable buffers w no problems
As others have said, USB 3 (and your MacBook) will be able to handle 22 tracks easily. My 2012 MBP certainly does. What specifically isn’t working with the aggregate device you’ve created? Is it dropping out? No signal?
Whatever software you’re using should have an Audio Preferences section where you select the aggregate device you created in Audio/Midi Setup as the audio interface that the software should use. Once this is done you you should then set its audio buffer size.
The bigger the buffer the more stable the audio signal, at the expense of a larger delay between a sound being generated outside the computer and and being heard inside the software. Aggregate devices increase buffer requirements so experimenting with audio buffers will usually fix issues relating to aggregate devices.
Here’s the instructions that apply to buffer settings in Ableton, but you can apply these instructions to any audio software (start from step 2):