I've finally got things set up the way that I want them with Reason on the Mac acting as midi clock source.
Having also properly worked out monitoring etc. (I was overcomplicating things as usual) I've been able to connect both the mixer and audio interface to give me more independent tracks to record on. It feels good!
pH 1 is going to be the first track to benefit from this setup.
I still want a new multi-channel audio interface that will allow me the flexibility of separately recording the individual outputs from the drum machines and having each synth on its own channel but I'll make do with what I've got. It's a marked improvement over how I had it set up before.