being honest i wouldnt really want it to be illumos-based anyways, KDE plasma to my knowledge only officially supports systemd which is linux-dependent, and Illumos has many regressions compared to the linux kernel and is not very robust for daily usage, has less FS support and other things.
theres a reason everyone uses the linux kernel, its the only one suitable for widespread usage on pcs and supports hardware, unless you can get amd, intel, nvidia, etc to support illumos its honestly not a good idea to switch off of what is already widely-supported
i would prefer if KaOS just stayed on linux because if KaOS used a different kernel i wouldnt be using the OS personally, using linux kernel makes the OS more suitable for daily usage.
the only real upside to using Illumos that i could see would maybe be ZFS support directly in kernel, but even then its not difficult to get ZFS on linux as the developer because you can just integrate it into the installer as an install option with compatible bootloader and kernel module
imo KaOS should just stay on linux, theres no reason to switch, its really the only one that works for all users, especially for gaming which imo is basically a necessity for computing for most pc users nowadays, especially now that gaming on linux is an official usecase for game devs and is well supported by valve