Do you know of a CEC adapter which can do HDMI2.1 4K120Hz? What has held me back from this setup is only being able to find 4K60Hz CEC adapters.

The CEC adapter shouldn't care? CEC is its own pin, everything else just goes straight through. (or maybe through twists).

You can also probably maybe do terrible things like CEC on one port and signal on another, it's mostly a bus.