I had trouble with the migration as well, and received insufficient space errors along the way. I believe the lack of space you are encountering is local, and not cloud based. Lightroom CC duplicates photos before uploading to the cloud.
Ultimately, I had to connect an external drive to my computer, and edit the "Local Storage" setting in lightroom cc preferences to use the external drive. Now I'm able to migrate, and am patiently getting through the long sync process of uploading originals.
The missing catalog was my own fault. to didn't notice that I changed the catalog place months before and delete to much in trying find space. Sorry adobe ;-)
But I I think anyway that adobe is working badly here. You think you get a new version with new possibilities - but it's not just an update. Most photographers have too many photos and too little space - hardly anyone has duplicate space. Adobe should inform us clearly before. Most will need to buy an external drive just for migration. Why is Adobe not able to realize the migration with the existing images? - they are transferred to the cloud anyway? Why is there no information that does not include all the features we already use (for example, HDRI from multiple images) - I would have expected more than less functionality. The publication of the new Lightroom CC seems to me premature.
Today, the next shock: 30,000 photos imported ... all cleanly arranged in folders. All photos are now sorted exactly by recording date. I can find the folder structure somewhere ... as I see this information is not even found in the metadata ... or information.
Had not Adobe at least the option that the existing folder name album to be imported or at least appear as a slogan?
I had a lot of photos afterwards in Photoshop worked and were imported and placed in the same orders .... they are now completely scattered by the temporal assignment ... I can not take it - that means an incomprehensible work, which was not absolutely necessary would be if the migration process were more lovingly designed :-(
The Lightroom Classic folder stucture will not copy to LrCC Ralf.
To retain this in some way Collections have to be created from the Folders before Migration.
There is some info here.......