Although I can’t offer any advice or help for OP, I came here to say I am facing this exact same issue.
- I followed the post-install guide, which got my mouse and keyboard etc working.
- I followed some of the stuff in this thread: HDMI through Thunderbolt 4 port not working on Redrix
- Also tried this: Modern Chromebook Setup Guide (with thunderbolt fixes)
- I tried generally playing around with ectool, with no luck.
Any support here from anybody would be greatly appreciated.