I would try to get things as I want with one device = one project. Then I will add 2 devices and then 3.
I would get rid of Audio Interface and use Apple Internal one firstly. If possible, no USB device plugged except Elektron request for the OB Session.
I set it up on my iMac Sierra 10.2.1 (i7) with 64 sample buffer and for the safety margin it depends about the DAW and differs with Analog HEAT
You can find more informations there :