i think i'm correct in saying the new mackies will only record to 2 traks at a time into the daw.
That is correct, having used them myself.
Most mixers with USB work that way, with only a few exceptions in the lower price range (more of the expensive stuff
does have multitrack interfaces built in).
Reminds me, I have a Mackie Onyx 1640 with the firewire interface that I need to finish rebuilding. Wonder if it'll work in BIAB...