Sorry, got caught up and forgot to update this.
After rebaking all posts, I got a different error message when trying the migration again.
I can’t find the backtrace anymore unfortunately.
I realized that despite of the error, it appeared that uploads were migrated to Backblaze and the posts updated.
I archived the local uploads and moved them elsewhere and have not heard any complaints since.
So in conclusion, yes, my issue was fixed.
Thanks to @pfaffman for the continued suggestions.
Now I have a complaint that various firewalls are blocking backblaze-hosted uploads, but that’s another matter.