If Illumos could be used as a fully functional kernel/base, that would automatically mean, start from scratch again, build everything, no systemd there (yet?).
But as answered in G+, when KaOS started, it was looked at first to build on Illumos, lack of hardware support like in the linux kernel, need to write all the tools (installer, package manager for a rolling release, etc) made it not a viable option at that time, despite it's many advantages.
Have you worked some with SmartOS? How are they handling updates now? Better hardware support? If SmartOS could be the base, then a switch might be possible, IF some decent tools come available.
I don't share your issues with systemd, everything has it's pros and cons, but to finally have so many distributions working together on one project is the biggest reason it is used in KaOS. It is something KaOS is also pushing for with an installer, getting many distros to work on one, unified installer.