So you should be able to turn off collision with the terrain itself to allow the user to freely move here, like this blog post explains: https://cesium.com/blog/2020/06/16/visualizing-underground/ (see links to code examples at the end).
I think that should work. The other route is implementing your own camera controller like here: https://sandcastle.cesium.com/index.html?src=Camera%20Tutorial.html. You’d have to handle collision yourself, such as perhaps by using scene.pickPosition each time the camera moves (or with the techniques Sean describes in the post you linked 3dTiles bounds and collision detection).