Nice summary and good reporting. Thanks.

Perhaps Tom Nowell's idea will pan out? Otherwise, I think there is a good argument to be made for a slower approach, along the lines suggested by Yoast, of only replacing the classic editor with the Gutenberg editor and not trying to shoot the moon on the first release. Such would allow a release sooner and get feedback / real-world usage of blocks for editing. It seems like we have been, as regards metaboxes, at the same place that we were back in the summer.