You did your research well here...I don't see anything that I can really fault in this build. As for the VCA implementation, go ahead and put the Quad VCA in, since VCA control is such a basic function and you'll want all of the basics in place before making the final decisions.
As for the later bits: the Nebulae isn't a bad choice, and another option to consider if you can find the space would be MakeNoise's Morphagene. Replacing the Rainmaker, though...I wouldn't. If you want an insanely-comprehensive and complex delay line, that module rivals even some of the most complex and celebrated outboard gear, and that makes it an optimal pick for its purpose.
Sequencing: have you also considered outboard? I'm planning for a KOMA Komplex, myself...the concept seems very open-ended, capable both in studio and live work. It would also free up space for additional modules in the cab itself, while defining the sequencer as a proper controller by placement. But there's plenty of possibilities beyond that one, like the Squarp Pyramid or Kilpatrick's Carbon.