Good news! All the topics were created! Bad news! none of the posts are connected to them, but hopefully that’s because the posts were created before the topics were.
That is odd. It was so sure I had an explanation. 
The 7 Million posts took only a couple of hours, but the <1M topics took like 4.
It’s an old machine (that they apparently bought just for the job?), and mysql is remote. Looking at htop there’s no obvious memory leak at the system level. I’ve wiped all the data and am rebuilding the containers to see if it’ll work this time.
Thanks so very much for your help.