And if I might, I think I have the perfect solution for that open 6 hp: Feedback's 106 Chorus. An excellent addition to an ambient-focussed system, with mono in and stereo outs, based on the Juno-106 chorus circuit. Paired with the uBraids, that would nail the effect processing situation nicely!
One other thing, also...the Toolbox is a nice idea, but having just one oscillator really undertaxes it. Instead, jamming that space with 2hp modules would allow for three sources (VCOs, or a mix of VCOs and some of their other things like the Vowel or Pluck) and a 2hp Mix to sum that down to the MMF for a nice paraphonic-type voicing. Yes, a bit more spendy, but the capabilities would really jump up at that point. You could also drop the Z4000 and go with a couple of Ladik modules; a C-214 replaces the ADSR capability of the Z4000, then tossing in a C-041 adds two AD/AR envelopes that can also function as LFOs if they receive a constant pulse from the Toolbox.